Evolution of an Eocene–Oligocene Saline Lake Depositional …dxy.cug.edu.cn/__local/E/CD/B1/ABBD17764EC33817ADBE206E... · 2018-07-23 · Evolution of an Eocene–Oligocene Saline

Journal of Earth Science, Vol. 25, No. 6, p. 959–976, December 2014 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-014-0499-2

Huang, C. J., Hinnov, L., 2014. Evolution of an Eocene–Oligocene Saline Lake Depositional System and Its Controlling Factors, Jianghan Basin, China. Journal of Earth Science, 25(6): 959–976. doi:10.1007/s12583-014-0499-2

Evolution of an Eocene–Oligocene Saline Lake Depositional System and Its Controlling Factors,

Jianghan Basin, China

Chunju Huang*1, Linda Hinnov1, 2 1. State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences,

China University of Geosciences, Wuhan 430074, China 2. Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore MD 21218, USA

ABSTRACT: The Upper Eocene–Lower Oligocene Qianjiang Formation of the Jianghan Basin in central China consists of a 4 700-m-thick lacustrine succession, containing 1 800 m of halite deposits. The maximum thickness of the formation is 4 700 m, and includes 1 800 m of halite. We have identi-fied eight third-order depositional sequences based on pinch-out and onlap stratigraphic patterns in 2-D and 3-D seismic data and well logs. The basin evolved from a deep to shallow under-filled lake during the Eocene–Oligocene interval. The main rock types are dark mudstones, halite, and siltstone/sandstone in the depocenter, and alternating mudstone and gypsum in shallower areas. The vertical succession indicates a strong sedimentary cyclicity. Depositional facies indicate the presence of two lake system types. Halite developed in a saline lake system, whereas clastic sediments were deposited in freshwater lake systems. The alternating sediment types indicate that the basin cycled repeatedly between saline and freshwater lake systems. This cyclicity was caused by availability of accommodation space that was controlled by a combination of climate change, tectonic subsidence and sediment supply; notably, the highest frequency cycles occurred at Milankovitch timescales con-trolled by the Earth’s orbital variations. The cyclic halite plays an important role in generating and preserving oil in the Qianjiang Formation of the Qianjiang depression. KEY WORDS: saline lake, depositional sequence, Eocene, Oligocene, Milankovitch cycle, Jianghan Basin, Qianjiang Formation.

0 INTRODUCTION

The Jianghan Basin of Central China is an important Ce-nozoic continental hydrocarbon-bearing basin (Fig. 1). The Eocene–Oligocene Qianjiang Formation, which comprises saline lacustrine sedimentary deposits in the Qianjiang depres-sion, one of the most productive areas of the Jianghan Basin (Wang D et al., 1998), has produced petroleum over the past forty years, mainly from structural traps. Today, exploration emphasis in the Qianjiang depression has shifted towards more subtle stratigraphic traps, which contain more than 42% of the oil reserves (Fang, 2006; Dai, 1997; Ye et al., 1997).

Lake water levels respond rapidly to climate change (Bomblies et al., 2001, Mann et al., 1995), in part because lakes involve much smaller volumes of water and sediment than oceans (Park et al., 2003; Betancourt et al., 2000; Vandervoort, 1997). Lacustrine stratigraphy provides a high-resolution record of geologic processes that are fundamentally different from those recorded by marine sedimentary deposits

*Corresponding author: [email protected] © China University of Geosciences and Springer-Verlag Berlin Heidelberg 2014 Manuscript received June 23, 2014. Manuscript accepted August 25, 2014.

(Pietras and Carroll, 2006). The stratigraphy of bedded evaporites provides clues about lake-level variations and saline lake expansion and contraction, and the interplay of climate, tectonics, sediment supply and accommodation space (Pietras and Carroll, 2006; Warren, 2006; Jones et al., 2001; Bohacs et al., 2000).

The halite-rich lacustrine facies of the Qianjiang For-mation were deposited during an underfilled basin phase (cf., Pietras and Carroll, 2006; Bohacs et al., 2000). Lake levels fluctuated in response to the balance between precipitation and evaporation (P/E) in a closed basinal setting (cf., Jones et al., 2001; Stine and Stine, 1990). This paper documents the basin-scale sequence stratigraphic framework and basin fill history of the Qianjiang depression throughout deposition of the Qianjiang Formation. The sequence stratigraphic framework developed in this paper provides a guide to the evolution of sedimentary facies across the Qianjiang depression throughout the severe hyperthermic conditions of the global Eocene greenhouse as well as the dramatic cooling at the Eocene–Oligocene transition.

In this paper, we discuss the sequence stratigraphic char-acteristics of the Qianjiang Formation and the accommodation changes that are thought to have controlled the depositional process. Halite deposition was the result of the concentration of saline lake water, and indicates lake level fall. Lake level fluc-

960 Chunju Huang and Linda Hinnov

tuation was an important factor that controlled the distribution of the lacustrine systems tracts in the basin, and is featured in the following sections. 1 GEOLOGICAL SETTING

1.1 Basin Setting The Jianghan Basin covers about 28 000 km2 in Hubei

Province, central eastern China, and contains 11 depressions, two major uplifts and two minor uplifts separated by two series of northwest and northeast trending faults (Wang D et al., 1998; Dai, 1997) (Fig. 1). The basin is an oil-gas-bearing Cretaceous–Oligocene rift-depression basin. Its basement con-sists of a >7 000 m thick, Precambrian (Sinian) to Early Trias-sic (mainly gypsum, dolomite-gypsum and limestone) marine succession, overlain by Middle Triassic–Jurassic non-marine foreland fluvial basin fill (You et al., 2006; Wiener et al., 1997). Halite-bearing units are distributed throughout the Qianjiang, Jiangling, Xiaoban and Yunmeng depressions (Fig. 1), but are replaced in the western Jiangling depression by

Figure 1. (a) Location and tectonic map of the Qianjiang depression and adjacent depressions in the Jianghan Basin, China; (b) sedimentary lithofacies of the Eocene Qianjiang Formation in the Qianjiang depression. Locations are shown for the two 2-D seismic lines (599 and 84.5), one 3-D seismic line (A–A’) and 19 wells (Z53, Z61, Z76, GS1, G24, G7, X15, S6, H64, N6, H58, H73, Y5, T70, H66, H4, H46, H24, QS8) used in this study (modified after Fang et al., 2003).

alluvial plain and fresh to brackish water facies (Grice et al., 1998; Dai, 1997).

The Qianjiang depression is located in the central portion of the Jianghan Basin, is 2 500 km2 in area (Fig. 1) (Dai, 1997; Peters et al., 1996), and is a half-graben that deepens to the northwest. It was affected by active movement of the Qianbei faults and became a depocenter during deposition of the Eo-cene Qianjiang Formation (Dai, 1997). The Qianjiang For-mation has a maximum thickness of 4 700 m and includes 1 800 m of halite. The latter consists of a succession of 193 rhythmites, each characterized by alternating halite and thin mudstone/shale or halite and gypsiferous mudstone (Fang et al., 2003; Grice et al., 1998; Wang R et al., 1998; Dai, 1997; Philp et al., 1989). The salt and gypsum were precipitated during hot and arid climate periods in a high salinity lacustrine environ-ment, while the mudstone and sandstone were deposited during more humid intervals with more freshwater influx (Grice et al., 1998).

Development of the Qianjiang depression was controlled by three tectonic cycles (Grice et al., 1998; Dai, 1997). The first cycle included thermal uplift and extension during the Early Cretaceous, followed by subsidence from the Late Creta-ceous to Early Eocene, when source rock and salt deposition took place within the Xingouzui Formation (Fig. 2). The se-cond cycle involved thermal uplift and extension during the Early–Middle Eocene followed by a depressional stage from the Late Eocene to Early Oligocene, and produced the major halite-bearing oil source rocks of the Qianjiang Formation. The third cycle involved uplift of the entire basin during the Hima-layan Orogeny from Late Oligocene (Dai, 1997); sedimentary deposits along the edge of the depression were eroded and the lake shrank. Subsequently, during Neogene and Quaternary times, fluvial and swamp environments characterized the area (Wang D et al., 1998; Dai, 1997).

1.2 Lithostratigraphy

The Eocene Qianjiang Formation is historically divided into four members based on lithology (Dai, 1997) and fossil assemblages (Ye et al., 1997). These are designated Eq4–Eq1 from the base upwards, as follows (Fig. 2). 1.2.1 Eq4 Member

The base of the Eq4 Member is marked by an unconformi-ty with the underlying Jingsha Formation (Dai, 1997), and is divided into lower and upper units. The Lower Eq4 Member consists of gray and dark-gray, laminated, thick mudstone beds, and oil shale interbedded with thin halite, with a thickness ranging of 173–2 218 m from basin margin to depocenter. The Upper Eq4 Member ranges in thickness from 100 to 700 m. Compared with the Lower Eq4 Member, there are thicker beds of halite near the base of the Upper Eq4 Member at the depocenter, and thicker sandstone beds at the northern and western basin margins. The Upper Eq4 Member in the southern part of the basin ranges in thickness from 100 to 350 m, and is comprised of glauberite-mudstone, gypsiferous mudstone, and mudstone-shale units interlayered with thin-bedded salts.

Evolution of an Eocene–Oligocene Saline Lake Depositional System and Its Controlling Factors 961

1.2.2 Eq3 Member The thickness of the Eq3 Member ranges of 150–640 m.

Near its base, thick beds of mudstone and oil-shale are interbedded with thin sandstone and siltstone throughout the basin, except for its southern part. In the lower-middle part of the member, thick beds (10–30 m) of halite occur in the depocenter and thick bedded sandstones at the north margin of the basin. From the middle to the top of the member, there are thicker beds (5–25 m) of mudstone and shale interbedded with thin sandstone/siltstone, and only a few thick halite beds in the depocenter. The southern basin has mudstone, gypsiferous mudstone, muddy-gypsum and mudstone/shale deposits with thin beds of salt and silty-mudstone. 1.2.3 Eq2 Member

The thickness of the Eq2 Member ranges from 110 to 700 m, depending on location in the basin. There are 24 halite-

bearing rhythmic beds, consisting of thick (up to 30 m) halite beds interbedded with dark-gray mudstone, gypsiferous mud-stone, mud-gypsum and oil shale in the basin depocenter; to-wards the top of the member, the lithology changes to thick mudstones interbedded with siltstone. At the basin margins, there are thick-bedded mudstones, shales and oil shales interbedded with thin siltstone, changing towards the top to relatively thick siltstone interbedded with mudstone. 1.2.4 Eq1 Member

This member has a conformable boundary with the under-lying Eq2 Member, and a total thickness ranging from 120 to 450 m. Throughout the basin, the lower part has thick mudstone/shale interbedded with gypsum-mudstone and thin salts, the middle part has medium thick (2–5 m) siltstone interbedded with gray mudstone, and the upper part has dark-gray mudstone, gypsiferous mudstone, mud-gypsum, and oil

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Figure 2. Stratigraphic column summarizing lithostratigraphy, depositional systems, source-reservoir-seal associations, strat-igraphic units, and the third-order depositional sequence (SIII) divisions in the Qianjiang depression in the Jianghan Basin. Recognition of the third-order sequences is based on the interpretation of 2D and 3D seismic data and well logging data, pre-sented in this paper. Sequences SIII1 and SIII2 correspond to the Lower Eq4 Member, SIII3 and SIII4 to the Upper Eq4 Member, SIII5 to the top of the Upper Eq4 Member and lower part of Eq3 Member, SIII6 to the uppermost part of the Eq3 Member, SIII7 to the Eq2 Member, and SIII8 to the Eq1 Member. Ages are according to Xu et al. (1995).


shale interbedded with halite. The member is overlain unconformably by the Jinghezhen Formation (Wang P et al., 2008; Wang D et al., 1998; Dai, 1997).

The dark-gray mudstone and oil shale from extremely an-oxic and highly saline depositional environments in the Eq3 and Eq4 members are major source rocks and reservoirs (i.e., inter-salt non-sandstone unconventional reservoirs); the dark-gray mudstone and related rocks in the Eq2 Member below 2 500 m are also major source rocks (Philp and Fan, 1987). 1.3 Geochronology

The Qianjiang Formation has been dated using K-Ar iso-topes (Xu et al., 1995): the base of Lower Eq4 is 43.0 Ma, the top of Lower Eq4 is 40.2 Ma, the top of Upper Eq4 is 39.2 Ma, the top of Eq3 is 37.7 Ma, and the top of Eq2 is 36.5 Ma (Fig. 2), with a relative error of 1.1%. This indicates an approxi-mately 6.5 Ma duration for the interval Eq4 to Eq2. Deposition-al rates based on these dates are as follows (depending on loca-tion, i.e. thickness): the Lower Eq4 Member is 0.062 to 0.79 m/ka; the Upper Eq4 Member is 0.1 to 0.7 m/ka; the Eq3 Mem-ber is 0.1 to 0.43 m/ka; the Eq2 Member is 0.092 to 0.58 m/ka; the Eq1 Member is about 0.07 to 0.28 m/ka. Accumulation rates may have been much faster for halite than for siltstone or mudstone in the Qianjiang depression (Dai, 1997). 2 MATERIALS AND METHODS

All data used in this study were collected by the Petrole-um Exploration and Development Research Institute, Jianghan Oil Field Ltd. (SINOPEC), and were acquired from 1965 to 2001. We constructed the sequence stratigraphic framework presented below from nearly 400 well logs, 21 2-D selected frame seismic profiles, and auxiliary profiles from 3-D seismic profiles. Some of the 2-D seismic profiles were interpreted using hard copies of paper sections; the rest were interpreted using the Schlumberger software Geoframe.

Our sequence stratigraphic interpretation applies the con-cepts of Vail (1987). This general sequence-stratigraphic mod-el consists of a depositional sequence, with a lowstand systems tract (LST), a transgressive systems tract (TST) and a highstand systems tract (HST) that occur at predictable posi-tions within an interpreted base level cycle, and have recog-nizable signatures in well logs and seismic data (Mitchum et al., 1993). The sequence stratigraphic interpretation of lacustrine deposits is based on sedimentological evidence for base level, i.e., lake level changes (Pietras and Carroll, 2006; Bohacs et al., 2000; Bourquina et al., 1998), which are controlled by varia-tions in accommodation space and sediment supply (Heller et al., 2001; Olsen et al., 1995). First we analyzed the well log sequences and systems tracts through interpretation of deposi-tional lithofacies from wireline logs calibrated to core and/or cuttings, and correlated the interpreted sequence boundaries and systems tracts between wells using parasequence stacking patterns and marker-beds. Second, we analyzed seismic se-quences and interpreted systems tracts by identifying disconti-nuities such as onlap, downlap, truncation, and toplap reflec-tion termination. Third, we synthesized the well log-based sequence stratigraphic interpretation, and tied well log se-quence surfaces to the seismic sequence surfaces with the well

log depth information, converting to seismic time using re-gional well log velocity scales for the Qianjiang depression (Wang D et al., 1998). Finally, we set up the sequence frame-work based on well log sequence correlations coupled with seismic sequences and systems tracts and the regional geologi-cal setting. For this paper we chose three seismic profiles (Line 599, Line 84.5, 3D A–A’), and 19 wells to present our interpre-tation (Fig. 1). 3 RESULTS 3.1 Depositional Facies

From the regional depositional setting, core observations, well logs and cuttings, five depositional facies have been rec-ognized in the Qianjiang Formation (Fang, 2006; Fang et al., 2003; Dai, 1997). Here we describe these facies and their char-acteristics in the seismic profiles.

3.1.1 Fan-delta

Fan-deltas consist of subaerial (alluvial fan), transitional (fan-delta front), and subaqueous (pro-delta) facies (McPherson et al., 1987). In the Qianjiang Formation, fan-deltas developed in the Eq4–Eq2 members in a narrow belt along the Qianbei steep fault zone, with sediment sourced from the northwest (Fig. 1). Here, the subaerial (alluvial fan) facies is characterized by chaotic, wavy, hummocky and discontinu-ous to moderately continuous seismic reflection patterns, in a narrow zone adjacent to the steep Qianbei fault sets (Fig. 3). This facies consists of poorly stratified and massive debris-flow deposits. The fan-delta front shows basinward progradation and downlap (Fig. 3a), and fair to moderate con-tinuity and subparallel seismic reflections (Figs. 3a, 3b). The fan-delta front deposits consist of mouth bar, subaqueous dis-tributary channel and sheet-like sand deposits, and are charac-terized by thickening and coarsening upward sequences and a spiky/blocky signature in spontaneous potential (SP) well logs (Figs. 3a, 4a, 4b). The subaqueous distributary channel depos-its are interpreted from well logs, and are characterized by blocky and fining upward SP log patterns and subparallel and high-amplitude seismic reflections (Figs. 3a, 4a, 4b). Finally, pro-delta deposits, adjacent to the basinal lacustrine mudstone and halite, consist of gray-black mudstone inter-bedded with marls and thin siltstone turbidites, and are characterized by high-value, relatively smooth SP patterns (Fig. 4a). 3.1.2 Sub-lacustrine/slope fan

Sub-lacustrine fan deposits are similar to those of basin floor fans in continental margin marine settings. In the Qianjiang depression, they occur in deep water settings at the base of the northern slope, adjacent to the Qianbei faults (Fig. 1b). Slope fans are mainly developed in the northwestern area of the depression. Sub-lacustrine fan and slope fan deposits are thick, dark grey-black mudstone/shale with thin siltstone de-posited by sediment gravity flow or high-density turbidity currents, the sub-lacustrine fans being formed by deep-water turbidites on the basin floor. In seismic profiles, sub-lacustrine fans have a mounded/hummocky shape and downlap to the underlying sequence boundary (Fig. 5c). They occur mainly in the Lower Eq4 Member, and are deeply buried and thus not


Figure 3. Excerpts of Line 599 (Fig. 7a) through Type A sequences tied to well logs, right is toward NW. (a) SIII3 to SIII6 with well logs Z61 and Z53 tied to the seismic profile; (b) SIII8 with well log Z53 tied to the seismic profile. The marked surfaces of each sequence and system tracts are converted from well log depth to time using the well log velocity scale of the northern Qianjiang depression. SP. Spontaneous potential. well drilled. The slope fans have boxcar/blocky and fining-upward SP log patterns (Figs. 3a, 4a). 3.1.3 Incised valley

Incised valleys are developed along the western slope, and are filled by thick beds of fine sandstone or siltstone interbedded with thin mudstone. The incised valley fills have blocky/boxcar SP log patterns with concave bases (Fig. 6b). Seismic profiles show landward onlap, basinward downlap, and truncated surfaces along the underlying sequence boundary (Figs. 5d, 6b).

3.1.4 Lacustrine Three lacustrine subfacies occur in the Qianjiang depres-

sion: fresh-brackish water facies, shallow saline facies and deep saline facies (Fang, 2006). Fresh-brackish water facies were deposited along the lake shoreline in northern and west-ern areas of the depression. Lithologies include gray to green-ish mudstone with calcium carbonate, and interbedded silt-stones and fine-grained sandstones with wavy bedding, cross-bedding, and bioturbation. The sand bodies are sheet-like or tabular thin beds, and generate spiky SP log patterns (e.g., ‘shallow lake’ on Fig. 4b) and moderate amplitude, good-continuity and parallel seismic reflections (‘shallow lake’, Fig. 3b). Shallow saline facies are developed in the south and


southeast of the basin, in the gently inclined shallow water area of the depression (Fig. 1). Lithologies here include interbedded gypsum/anhydrite, gypsiferous mudstone, glauberite-mudstone and mudstone. These were deposited below wave base, display horizontal lamination and contain abundant ostracods, gastro-pods and algae. This facies is indicated by an irregular/spiky SP/RT log pattern (Fig. 4c). Deep saline facies occur in the depocenter and contain interbedded halite and dark gray mudstone/shale in successions up to 1 800 m thick, with boxcar/blocky and fining-upward GR log patterns (Figs. 4d, 4e), and moderate to strong amplitude, good continuity and

parallel seismic reflections (Fig. 6c). The mudstones/shales have organic rich, horizonal laminations and are the main source rock of the depression. 3.1.4 Saline lake density current deposits

Saline lake density current deposits are caused by an in-flux of freshwater carrying fine sand and mud that floats on top of saline water. When the suspended sediment load exceeds the density of the underlying saline water, it sinks. The distribution of these unique, lenticular deposits is thus controlled by densi-ty and diffusion mechanisms. These deposits are found within

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Figure 4. Well logs illustrating the three sequence types. (a) and (b) Type A sequences, SIII3 and SIII5 clastic sequences (interbedded sandstone-shale cycles) in the northern and western slope, from wells Z61 and Z53; (c) Type C shallow saline lake sequence SIII5 from Well QS8; (d) and (e) Type B sequences SIII6 and SIII7 from Well Y5 with inter-bedded salt-shale cycles in the saline lake center. The thick halite inter-bedded with thin shales occur at ~100-ka timescales. MFS. Maximum flooding surface; LST. lowstand systems tract; TST. transgressive systems tract; HST. Highstand systems tract.


Figure 5. (a) 2-D seismic Line 84.5 (SW-NE); (b) seismic sequence stratigraphic interpretation; (c) sublacustrine fan in Line 84.5; (d) SIII7–SIII8 sequence boundary unconformity and incised valley fill. Fm.. Formation. thick mudstones/salts, and have complex, fining upward and coarsening upward structures and a blocky/bell-shaped GR log pattern (Fig. 4d, between 2 650–2 840 m). Saline lake density current deposits can be divided into three types, based on loca-tion: coastal, deep saline lake and distal saline lake deposits. Coastal density current facies occur in front of the fan-delta (Fig. 7). Deep saline lake density current deposits occur epi-sodically in the depocenter from Eq4 to Eq3 (e.g., Fig. 5c, GS1, 2 730–3 010 m; Fig. 4d, Y5, 2 650–2 840 m). Distal saline lake density current deposits, including lenticular sand bodies, were recognized in the southern gentle slope of the depression, which indicates that the density currents traveled a long dis-tance across the depocenter and were deposited in the southern, shallow water areas (Fig. 7) (Fang, 2006). 3.2 Sequence Types

We recognize three sequence types in different areas of the Qianjiang depression as follows. 3.2.1 Type A sequence (fresh water/clastic)

This sequence type occurs in the Qianbei steep slope zone on the northern and northwestern margin of the depression; the zone is the expression, in large measure, of the Qianbei fault, which was also responsible for sediment being sourced from the northwest and for the creation of accommodation space.

Study of cuttings and well logs shows that fresh water with terrigenous clastic sediment was abundantly supplied to the saline lake (Wang D et al., 1998; Dai, 1997). Consequently, the stratigraphic sequences in this area consist of clastic sequences in a narrow belt (Fig. 8a, Table 1). Terrigenous sediment input and freshwater influx were such that halite was poorly devel-oped. The sequences are mainly composed of alluvial fan, fan-delta and shallow-lacustrine depositional systems (Figs. 3, 4a, 4b, 6b). The LSTs consist of sub-lacustrine fan (Fig. 5c), slope fan (Figs. 3a, 4a), incised valley fill (Figs. 5d, 6b) and shallow to deep lacustrine deposits, and comprise thick beds of sandstone/siltstone interbedded with thin mudstone. Slope fans show basinward progradation (Figs. 3a, 4a), and landward pinchout of other LST deposits is evident in seismic sections (Figs. 3a, 5b, 6b, 9b). The TSTs consist of shallow lacustrine deposits comprising thin sandstone/siltstone interbedded with thick dark mudstone/shale, and exhibit retrogradational and aggradational stacking patterns (Figs. 4a, 4b). Seismic reflec-tors are of moderate to high amplitude and moderate continuity, and are subparallel (Figs. 3, 6b). The HSTs include fan-delta and shallow lacustrine deposits with aggradational and basinward progradational stacking patterns (Figs. 3a, 4a, 4b, 6b). They are associated with moderate to high amplitude, moderate continuity and subparallel seismic reflections (Figs. 3, 6b).


3.2.2 Type B sequence (deep saline lake/halite-rich) This sequence type occurs in the depocenter of the de-

pression and is characterized by Wells GS1 and Y5 (Figs. 5a, 5b, 9a, 9b). Type A sequences change gradually and laterally into Type B sequences towards the deep depression depocenter (See Table 1 for comparison of the two types of sequence). In the depocenter, saline lacustrine facies consist of thick halite and dark gray mudstones interbedded with thin density current

sands (turbidites). The LSTs consist of deep saline lake facies with thick halite interbedded with thin dark mudstones (Figs. 4d, 4e), and have strong parallel seismic reflectors of high amplitude and good continuity (Fig. 6c). The TSTs consist of deep lacustrine facies and turbidites, saline lake density current deposits and thin halite/siltstones interbedded with dark mudstone/shale (Figs. 4d, 4e), and have moderate to high am-plitude, moderate continuity and subparallel-parallel seismic

Figure 6. 3-D seismic line (A–A’) and excerpts through Type A and Type B sequences tied to well logs. GR. Gamma ray; SP. spontaneous potential; RT. resistivity; right is toward NE. (a) 3-D seismic line (A–A’) in SW-NE direction; (b) SIII3 to SIII6 LST pinch out landward and have delta front characteristics in the western slope of the Qianjiang depression; (c) SIII6 and SIII7 are tied to logs at well GS1. Lithology key see Fig. 8.


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Fault and depression

boundary line

Well location

Proximal fan-delta front

Distal fan-delta front

Coastal saline lake/fan-delta

front density current deposits

Saline lake density current

lenticular sandbody

Distal saline lake density

current deposits

Shoal-water delta-fan front

H24

H64G7

G24

GS1

X15

S6

T25

Y6

Y5Z53Z61

QS8

196 40'o

196 50'o

196 60'o

196 70'o

196 80'o

196 90'Eo

33

80

'No

33

70

'o

33

60

'o

33

50

'o

0 10 km

Figure 7. Depositional facies distribution of the LST in SIII3.

Table 1 Siliciclastic vs. saline lakes in sequence stratigraphy

Sequence stratig-raphy unit

Type A (siliciclastic) sequence Type B (saline-lakes) sequence

Sequence boundary

Significant erosional unconformities and their correlative conformities; above this surface are coarse clastic rocks or correla-tives

The sequence bottom and adjacent deposits char-acterized by thick halite

Lowstand systems tract (LST)

Low accommodation space, water range limited, progradational parasequence sets with thick sandstone interbedded with mudstone

Low accommodation space, limited basin water range and volume, high water salinity, halite well developed and thick

Transgressive systems tract (TST)

Increasing accommodation space, enlarg-ing water range, retrogradational parasequence sets with interbedded mud-stone and thin sandstone

Increasing accommodation space and water vol-ume, lower water salinity, retro-gradational or aggradational parasequence sets with interbedded thick mudstone and thin halite or thin sandstones/ silt

Highstand systems tract (HST)

Accommodation space decreasing, aggradational to progradational parasequence sets with sandstone and mudstone association

Water range still large, but accommoda-tion space and water volume decreasing, increasing water salinity, progradational or retrogradational parasequence sets, interbedded thick mudstone and thin sandstones in the early HST, halite rela-tively thick during late HST

Maximum flood-ing surface (MFS)

Separates the TST from the HST, radioac-tive and often organic rich shales

Separates the TST and HST, thin shale

reflections (Fig. 6c). The HSTs include saline lake deposits and thin density-current sandstones, and comprise thin/thick halite/ siltstone interbedded with dark mudstone/shale. They are asso-ciated with strong parallel seismic reflections with high ampli-tude and good continuity (Fig. 6c).

3.2.3 Type C sequence (shallow saline lake) This sequence type is developed in the southern and

southeastern area of the depression (Fig. 1). Sequences consist of shallow saline lake facies with abundant evaporites, and both sequences and beds are relatively thin compared with Type B sequences in the depocenter. Only the TSTs and HSTs


are developed; LSTs pinch out towards the south and southeast of the basin (Fig. 9b). The TSTs and HSTs are difficult to dis-tinguish because the lithologic facies are similar for all of the Eq1 to Eq4 members (Fig. 4c). Lithologies include interbedded gypsum/anhydrite, muddy-gypsum, gypsum-mudstone, glau-berite mudstone and thin mudstone deposited on a gently in-clined, shallow-water platform (Figs. 4c, 9b, 10). 4 DISCUSSION 4.1 Basin Evolution

Eight third-order depositional sequences, designated SIII1–SIII8, and their component systems tracts were recognized in the Qianjiang Formation (Fig. 2), based on reflection termina-tions interpreted from the seismic lines, well-cuttings and wireline logs. The sequences are dominated by the repetition of lake expansion and contraction, expressed as high-frequency depositional cycles in basin evolution. Paleotopography con-trolled deposition of the sequences; sequences SIII3–SIII8 are also affected by salt mobilization in SIII1–SIII2 (analogous to process-es described in Madof et al., 2009) (Fig. 9b). We describe each

sequence below, mainly focusing on Type A and Type B se-quences because these are the most important economically, having produced most of the hydrocarbons from the basin. 4.1.1 Sequence SIII1

SIII1 forms the Lower Eq4 Member, deposited during the early rift phase of Qianjiang depression evolution (Fig. 2). Facies indicate that the area was a deep-water, under-filled lake. Under the influence of active faulting, the basin underwent significant subsidence and formed a NW-SE trending half graben, deep in the north and shallowing southwards. The SIII1 sequence boundary (SB1) is associated with local erosional truncations and a strong, fluctuating reflection (Figs. 9c, 9d). The LST of SIII1 was deposited in the depression center (Fig. 11), mainly as sublacustrine fans with onlap and downlap rela-tionships with SB1 (Fig. 5c). The LST is overlapped by the TST towards the NW, towards the Qianbei fault, and is over-stepped towards the SE (Fig. 9b). The TST of SIII1 consists of thin siltstones interbedded with thick mudstones (Type A se-quence) in the northern depression, and thick mudstone inter-

Figure 8. (a) Well-log correlation of SIII5 HST along Line 599 (SE direction to the right). To the north, fan-delta and delta-front sandstones extended directly into the lake depocenter, changing abruptly into halite deposits; (b) well log correlations of SIII5 HST along the 3-D seismic Line A-A’ (NE direction to the right). In the western slope, the delta-front sandstones bodies change gradually to mudstone and then halite toward the lake depocenter. Both profiles show well-developed delta-front and deepwater lacustrine reservoirs in SIII5.


Figure 9. 2-D seismic Line 599 and sequence stratigraphic interpretation. Right is toward NW. (a) The seismic line; red boxes indicate excerpts shown in (c) and (d) and Fig. 3; (b) sequence interpretation showing the stratal architecture of SIII1 to SIII8 of the Qianjiang Formation; the marked surfaces and subsurfaces of each sequence have been converted from well log depth to time using average velocity; (c) excerpt from (A) showing undulating SB1 reflector; (d) excerpt from (a) showing SIII1 to SIII3 and well log tied to the seismic profile. Lithology key see Fig. 8. QJ. Qianjiang; Fm.. formation. bedded with thin halite (Type B sequence) in the depocenter. In the HST of SIII1, the ratio of sandstone/siltstone: mudstone or halite: mudstone increases and the parasequence stacking pattern changes from aggradation to progradation (Fig. 9d).

4.1.2 Sequence SIII2

During SIII2, which forms the upper part of the Lower Eq4 Member, the areal extent of deposition gradually in-creased. SB2 is associated with salt flow and a wedge-shaped salt rock body (Figs. 9b–9d). The sequence boundary is not well defined at the northwestern slope break due to erosion and exposure, and it there correlates with an alluvial fan/subaerial fan-delta facies (Fig. 3a). The LST of SIII2 was deposited in the depocenter. It extends towards the southeast of the depression (Fig. 11) and contains more salt than SIII1. The TST deposits consist of dark mudstones interbedded with siltstones/salt in the Type A and B sequence areas. During the HST, more salt developed in the Type B sequence area (Fig. 9d), and fan deltas developed in the Type A sequence area (Fig. 3a). Only the TST and HST developed in the Type C sequence area (e.g., Fig. 9b, southeastward from between

boreholes X15 and S6).

4.1.3 Sequence SIII3 This sequence forms the lower part of the Upper Eq4

Member (Fig. 2), and was also deposited during the rift phase of Qianjiang depression evolution. Extensional faulting per-sisted along the northern border, and the lake underwent gradual expansion and deepening. There is evidence for more fresh water and terrigenous sediment influx into the depres-sion compared with SIII1 and SIII2. The LST area enlarged considerably (Fig. 11) with more sandstone and siltstone than SIII1 and SIII2, including widely distributed density current lenticular sand bodies (Fig. 7). SB3 is a continuous and strong reflection surface, and lowstand fans pinch out to-wards the west and northwest (Fig. 6b), and the slope fan progradation southeastwards into the basin in SIII3 (Fig. 3a). In the Type A sequence area, the LST consists of thick silt-stone interbedded with mudstone that form slope fans with a prograding parasequence stacking pattern (Figs. 3a, 4a); the TST consists of thick dark mudstone with siltstone (Fig. 4a) showing a retrogradational stacking pattern; the HST is a fan-


delta depositional system, with an aggradational pattern in the early HST changing to a progradational pattern in the late HST (Fig. 4a). In the Type B sequence area, the early LST comprises thick halite (~10 m) with dark mudstone, and the late LST is mainly thick dark mudstone inter-bedded with thin siltstone and salt (1–3 m) (Figs. 10, 12). The lithology of the TST is similar to the late LST (Figs. 10, 12). The li-thology of the early HST is also similar to the late LST but with thin salt beds in an aggradational pattern; salts in the late HST increase in thickness (from <1 to 1–10 m) and are interbedded with dark mudstone (Figs. 10, 12). The TST and HST are relatively thin towards the southeast depression, in the Type C sequence area (Fig. 10).

4.1.4 Sequence SIII4 SIII4 forms the upper part of the Upper Eq4 Member. Ex-

tensional faulting remained active but with decreased intensity, and the lake gradually expanded to the southeast (Fig. 11). SB4 is a continuous and strong reflection surface indicating a lithologic change in the depocenter from siltstone and mud-stone in the top HST of SIII3 to salt at the base of the LST of SIII4. The LST pinches out towards the northern and western slopes (Figs. 3a, 6b). In the Type A sequence area, the LST and TST are mainly gypsum-mudstone, oil shale and mudstone interbedded with a few thin siltstone beds. A fan-delta system is developed in the HST, with sandstone/siltstone increasing in thickness the late HST (Figs. 10, 12); the parasequence

Figure 10. Well-log correlation from SIII3 to SIII8 along Line 599 showing Type A sequences in the northwest, Type B se-quences in the lake depocenter, and Type C sequences in the southeast.


0 10 km

S 1 LSTIII SIII2 LST SIII3 LST

SIII4 LST SIII5 LST SIII6 LST

SIII7 LST SIII8 LST

Well location

Fault

Uplift boundary Sand pinchout line

Delta fan front

trap area

Lowstand fan pinchout

trap area

Density-current sandstones

(turbidites) trap area

H66H4

H24

H46

X15

G7

S6

QS8

Y5

GS1

G24

Z76

H64N6

H58

T70

196 40'o

196 50'o

196 60'o

196 70'o

196 80'o

196 90'o

197 00'o

197 10'Eo

33

90

'No

33

80

'o

33

70

'o

33

60

'o

33

50

'o

33

40

'o

Z53

H73

Figure 11. Areal distribution of SIII1 to SIII8 LSTs and stratigraphic trap prospects. stacking pattern changes from aggradational in the early HST to progradational in the late HST. In the depocenter of the Type B sequence area, the LST mainly developed as thin-medium beds (1–5 m) of halite interbedded with dark mud-stone (Figs. 10, 12). The TST has similar lithologies to the LST, but with more dark gray mudstone and shale. In the early HST, thick dark mudstone/oil shale is interbedded with thin halite/siltstone (1–2 m), overlain by medium to thick halite beds (2–12 m) interbedded with thin mudstone/oil shales in the late HST (Figs. 10, 12). The Type C sequence is similar to SIII3 (Fig. 10). Compared to SIII3, SIII4 indicates less terrigenous sediment influx into the depression and possibly a drier climate (i.e., more halite). 4.1.5 Sequence SIII5

SIII5 occurs at the top of Upper Eq4 and in the Lower Eq3 Member, and was deposited during the early post-rift stage of the Qianjiang depression (Fig. 2). The intensity of extensional faulting decreased, although the lake continued to expand and deepen. The distribution area of the LST is slightly larger than that of SIII4 (Fig. 11), and there was more fresh water and terrigenous sediment influx into the depression. SB5 is a con-tinuous strong reflection surface in the depocenter (Fig. 6b) due to a lithologic change from salt in the late HST of SIII4 to mudstone/sandstone in the early LST of SIII5. Incised valley sandstones occur along SB5 and onlap/pinch out towards the western slope (Fig. 6b); they are important reservoir prospects. In the Type A sequence area, the LST developed lowstand

fan/slope fans that onlap/pinch out towards the western and northern slopes (Figs. 3a, 6b) and comprise medium beds (1–10 m) of coarse sandstone (Fig. 4b). The TST consists of thick mudstone interbedded with relatively thin (0.5–3 m) siltstone and sandstone, and exhibits retrogradational and aggradational stacking patterns (Fig. 4b). In the early HST, prodelta deposi-tion of thick mudstone interbedded with thin (1–2 m) sand-stone and siltstone with an aggradational stacking pattern was followed in the late HST by fan-delta front deposition, with medium to thick (2–15 m) sandstone/siltstone with a progradational pattern (Fig. 8). In the Type B sequence area, the LST and TST are mainly thick, dark mudstone/oil shale interbedded with thin beds of halite/siltstone (0.5–1 m) and some thick (3–5 m) siltstones (density current deposits). The early HST deposits are similar to those in the TST, but there are thick beds (5–20 m) of halite interbedded with dark gray mudstone/oil shale in the late HST (Fig. 12). The Type C se-quence is similar to those of SIII3 and SIII4 (Fig. 4c). Because the slope was steep in the northern part of the depression, the Type A sequence depositional zone was quite narrow, with an abrupt lateral transition into a Type B sequence (Fig. 8). 4.1.6 Sequence SIII6

This sequence forms the Upper Eq3 Member, deposited during the post-rift stage of the Qianjiang depression (Fig. 2). The lake was still expanding and deepening, and the distribu-tion area of the LST increased (Fig. 11). SB6 is also a continu-ous strong reflection surface in the depocenter, indicating a


lithologic change from mudstone/sandstone of the late HST in SIII5 to salt in the early LST of SIII6 (Figs. 3a, 5c). Toplap, onlap and pinch out are seen in reflectors along SB6 on the western slope (Fig. 6b). In the Type A sequence area, the early LST consists of mudstone/oil shale interbedded with salt or gypsum-salt/siltstone, and late LST slope fan deposits consist of thick (3–15 m) beds of sandstone or siltstone on the western and northern slopes that pinch out towards the basin margin with a progradational stacking pattern towards the basin center (Fig. 6b). The TST developed thick mudstones interbedded with thin beds (1–3 m) of siltstone/sandstone of shallow lake facies, in retrogradational and aggradational stacking patterns (Fig. 10). Aggrading sandstone/siltstone in the early HST gives way to fan-delta systems with progradational stacking patterns in the late HST (Fig. 3a). In the Type B sequence area, the LST

consists of mainly thick beds (10–30 m) of halite interbedded with thin dark mudstone and oil shale (Figs. 4d, 6c). The TST consists of thick mudstone interbedded with thin siltstone and a few thick (up to ten meters) density current lenticular sand bodies (Figs. 4d, 6c). The early HST consists of mudstone interbedded with siltstone with a progradational stacking pat-tern; halite increased in the late HST (Figs. 4d, 6c). The Type C sequence is similar to that in SIII5 (Fig. 10). 4.1.7 Sequence SIII7

This sequence developed in the Eq2 Member during the post-rift stage of the Qianjiang depression (Wang D et al., 1998; Dai, 1997). Lateral thickness changes of the types A–C se-quences are smaller than those in the lower sequences due to the creation of less accommodation space. The area of the LST

Figure 12. Well-log correlation from SIII3 to SIII8 along Line 84.5 showing Type A sequences in the southwest, and Type B sequences in the lake depocenter.


increased relative to the lower sequences (Fig. 11). SB7 is a very strong and continuous reflection surface due to strongly contrasting lithologies (siltstone/sandstone at the top of SIII6 to salt at the base of SIII7) and the primary seismic reflections are parallel (Fig. 6c). Onlapping reflection terminations occur only in the western slope areas (Fig. 6b). In the Type A sequence area, the LST and TST consist of thick mudstone, gypsum-mudstone, thin salt and oil-shale interbedded with a few silt-stone beds; siltstone increased in thickness in the HST, which exhibits a progradational stacking pattern in Well Z61 (Fig. 10). The Type B sequence area consists of dominantly thick halite in the depression center (Figs. 4e, 12) and reflects development of the most saline waters during evolution of the Jianghan Basin. The Type C sequence is similar to that of the lower sequences (Fig. 10). 4.1.8 Sequence SIII8

SIII8 developed during thermal subsidence (depressional) phase (Wang D et al., 1998; Dai, 1997). Lateral thickness changes are again much smaller than in SIII7. SB8 is associated with a very strong reflection surface; incised valleys and trun-cations are found along the western and eastern slopes (Figs. 3b, 5d). The LST is developed through most of the depression (Fig. 11). The early LST has some thick halite beds precipitat-ed in the saline lake center in the Type B sequence area (Fig. 12), but in later stages of the LST and in the TST and HST, deposits are mainly mudstone and shale interbedded with silt-stone/sandstone throughout the Type A and Type B sequence areas (Figs. 10, 12). The Type C sequence consists of mud-gypsum and glauberite-mudstone in the TST and early HST, but the lithologies change to mudstone and gypsum-mudstone interbedded with thin siltstone in the late HST, indicating that the areal extent of the saline lake was decreasing. The lake became shallow during deposition of late SIII8, the lake type changing from underfilled to balanced.

Above the Qianjiang Formation, SIII9 and SIII10 formed in the Jinghezhen Formation during the uplift phase, which was a balanced lake period; erosion and truncation terminations oc-cur in the western, northern and northeastern slopes (Figs. 3b, 5d). Subsequently, the basin became overfilled and lacustrine deposition ceased. 4.2 Halite in Lacustrine Sequence Stratigraphy

The source of the abundant salts in the Qianjiang For-mation is not clear. The fresh and brackish water facies and the absence of marine fossils indicate strong continental influences. Faulting and volcanism were active (Xu et al., 1995), and may have introduced geothermal brines into the basin to enhance the lake’s saturation state. The amount of brine that would have been required to precipitate the great thicknesses of halite (up to 1 800 m) seems to indicate that geothermal sources alone might be inadequate, although non-marine evaporites can reach many hundreds of meters in thickness (Hardie, 1984). However, recently, Wu (2007) suggested that the narrow, N-S Palaeogene seaway of East China extended as far south as Jianghan Basin. Marine fossils are known from the Early Eo-cene Xingouzui and Jingsha formations, just below the Qianjiang Formation (Fig. 2). The coeval Shahejie Formation

in the Dongpu depression, which lies to the north of the Jianghan Basin, does contain marine and estuarine fossils throughout the Eocene and into the Early Oligocene. Thus, it is possible that the Jianghan Basin may have experienced episod-ic marine inundations throughout the Eocene that replenished the supply of salt to the Qianjiang depression.

The halite-rich Type B sequence of the Qianjiang For-mation differs from a basic siliciclastic lacustrine sequence (Table 1). A key issue is how the thick halite facies fits into the systems tract sequence. In this case, shallow water evaporite deposition (Einsele, 2000) cannot explain the thick halite, which precipitated only in the depression center. The basin subsidence rate must have been faster than the deposition of the halite, and the Qianjiang depression was very deep when the salt was deposited (Wang D et al., 1998; Dai, 1997). Thus, the underfilled lake stage encompassing SIII1–SIII6 (Eq4 and Eq3 members) could be explained as a deep-water genetic model of a saline lake (Einsele, 2000; Schmalz, 1969). During the thermal extension and subsidence phase of Eq2 (SIII7) time, there was continued uplift, the lake shrank, brine concentration levels reached a maximum, and the salt deposits transitioned to shallow water facies (Dai, 1997). At the end of Eq1 (SIII8) time, the saline lake became dilute until the basin was completely filled. Hence, salt was deposited in both deep water and shal-low water environments of the Qianjiang Formation, but at different times.

There was increasing siltstone and mudstone deposition and decreasing salt deposition during the late LST and TST of SIII3, SIII5 and SIII6 when base/lake levels were rising. For example, during TST deposition, accommodation space gradu-ally increased, and when lake level reached the maximum flooding surface and water volume and accommodation space were at a maximum, water salinity was at a minimum and hal-ite deposition was much reduced or absent; lithologies were dominantly thick mudstone interbedded with thin sandstone beds. However, salt deposition increased during the base level fall stage in the HST and early LST of SIII3, SIII5 and SIII6, when salinity increased and water volume decreased. The thickest halite dominates deposits of SIII7 in the Type B se-quence area, indicating the most saline water during deposition of the Qianjiang Formation. During this time, evaporation greatly exceeded fresh water input/precipitation, suggestive of a very dry and hot climate. Hence, the development of halite in different systems tracts (Table 1) is directly controlled by wa-ter salinity and not by water depth.

Halite beds are very good seal/cap rocks and play an im-portant role in preserving oil and gas in underlying sandstone reservoirs and inter-salt non-sandstone unconventional reser-voirs. SIII3, SIII5 and SIII6 are the most productive sequences in the Qianjiang Formation (Fang, 2006) most likely due to halite. The dark mudstone and shale between the halite rich intervals in SIII1–SIII7 (below 2 500 m) are the main source rocks in the formation, and serve also as the main oil and gas reservoirs (Fig. 2) (Philp and Fan, 1987). 4.3 Diagenesis and Deformation

Diagenesis of the Qianjiang Formation involved compac-tion, dissolution and salt recrystallization. Salt dissolution oc-


curs with changes in temperature and pressure; the dissolved salt subsequently recrystallizes. With recrystallization, the salt body expands and deforms the sedimentary structure. There are 1 200 m of halite and 300 m of soft mudstone in SIII1–SIII2 (Lower Eq4 Member) in the depocenter. Experiments (Fang, 2006; Dai, 1997) show that salt is subject to temperature and pressure conditions that result in softening and plastic flow when buried to 1 500 m. In the Qianjiang depression, most of the Lower Eq4 Member occurs below 3 000 m, where salt den-sity is 2.13 g/cm3 and soft mudstone density is 2.25–2.28 g/cm3, i.e., less than sandstone density of 2.40–2.68 g/cm3. Thus, sandstone density in SIII3 is greater than the densities of salt and soft mudstone in the underlying SIII1–SIII2. Under these conditions salt flowed, driven also by the difference in sedi-mentary thickness between the footwall and hanging wall of the faults, and became allochthonous (Figs. 5a, 9a). Therefore, the character of depositional sequences (SIII3–SIII8) was further controlled by post-depositional extension and salt deformation (cf., Madof et al., 2009). 4.4 Origin of High Frequency Depositional Cycles

The Qianjiang Formation has a rhythmic stratigraphic ar-chitecture characterized by alternating deposits of terrigenous clastic sedimentary rocks and chemical precipitates, arranged in meter- to decameter-scale cycles of alternating salt and mudstone or salt, mudstone and sandstone. This indicates that the sediments were deposited in alternating saline to freshwater lakes controlled by dry to humid climate changes (Grice et al., 1998; Wang D et al., 1998; Dai, 1997).

High-frequency lake level fluctuations reflected in a strong sedimentary cyclicity throughout the formation, particu-larly in SIII6 and SIII7, appear to have been driven by astronom-ically forced climate change, which is well documented in ancient lacustrine systems (Abels et al., 2009; Machlus et al., 2008; Vollmer et al., 2008; Ashley, 2007; Olsen and Kent, 1999). The cyclic, salt-rich interval, especially in SIII7, is char-acterized by hierarchical 5–10 m and 25–50 m depositional cycles suggestive of precession-eccentricity forcing (Huang and Hinnov, 2010).

The halite-rich deposits in SIII6 and SIII7 are marked by strong high-frequency cyclicity, with decameter-scale (25–50 m) halite deposits with siliciclastic units intercalating at the meter-scale (5–10 m) (Figs. 4d, 4e). Geochronological con-straints indicate depositional rates ranging from 6 to 79 cm/ka. These rates suggest that the “bundled” halite-siliciclastic cycles occur at a ~100-ka timescale, and the intercalations at ~20-ka timescales. This compound cyclicity points to a dominant pre-cession-eccentricity forcing of climate and deposition. The cycling facies indicate alternations between hot/dry climates with low base levels (halite), and cool/humid climates with high base levels (siliciclastics). The cyclicity is captured at high-resolution by the well logs (e.g., Fig. 4). These ~100-ka short eccentricity cycles correspond to fouth-order sequences (parasequences) and ~2 Ma long eccentricity cycles should correspond to third-order sequences (Tian et al., 2014; Liu, 2012; Boulila et al., 2011). Further study of these logs may lead to the definition of a continuous astrochronology for the Qianjiang Formation (Huang and Hinnov, 2010). The

astronomically-forced cyclicity would be expected to express the unique modulations of the Late Eocene–Early Oligocene orbital eccentricity, and possibly allow correlation into the marine record (Pälike et al., 2006, 2001).

Future work in understanding this remarkable geologic archive includes identification of the brine source(s) of the halite, and researching the origin of the high-frequency halite cycles, their relationship to the third-order sequences, and inte-gration with the global climate record. 5 CONCLUSIONS

This study has focused on the sequence stratigraphic framework of the Upper Eocene–Lower Oligocene lacustrine Qianjiang Formation (Jianghan Basin, China). A unifying strat-igraphic framework was defined from interpretation of base level variations, and recognition of a succession of third order sequences in the formation. The base level variations track repeated lake expansions and contractions for at least 6.5 mil-lion years. Principal results are as follows.

1. The Qianjiang Formation evolved from a deep-water underfilled lake basin to a shallow-water underfilled lake basin, with halite-rich deep-water deposits in the Eq4 and Eq3 mem-bers, and halite-rich shallow-water deposits in the Eq2 and Eq1 members.

2. Eight third order sequences, SIII1–SIII8, were recog-nized in the Qianjiang Formation, based on analysis and inte-gration of an extensive 2-D and 3-D seismic dataset and more than 400 well logs. Sequences are classified into three types that are laterally restricted: Type A (fresh water/clastic), Type B (deep saline lake/halite-rich), and Type C (shallow saline lake).

3. The origin of the halite has not yet been firmly estab-lished. The depositional setting of the Qianjiang Formation is continental lacustrine, but may have been influenced by the Palaeogene seaway of East China. Halite-rich deposits devel-oped in different systems tracts in this saline-lake system, and were controlled by changes in accommodation and lake/base levels, indicating dry and hot climate conditions, reduced fresh water input and anaerobic conditions. Halite as a seal rock plays an important role in preserving oil and gas in the Qianjiang Formation reservoirs.

4. High-frequency alternating halite/mud facies succes-sions are associated with repeated expansion and contraction of an underfilled lake. These cycles (25–50 m) may have been driven by astronomically forced climate change. ACKNOWLEDGMENTS

This study was supported by the National Natural Science Foundation of China (No. 41322013), the Program for New Century Excellent Talents in Universities (No. NCET-11-0723), the National Key Basic Research Development Program of China (No. 2012CB822003), the Programme of Introducing Talents of Discipline to Universities (No. B14031) and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUG110611). We thank Jory Pacht, Stewart Molyneux and Philip Richards for numerous, very helpful suggestions for earlier versions of this manuscript. Paul Weimer of the University of Colorado


(Boulder) provided generous assistance with the presentation of this work. We also thank Kaiyuan Chen, Sitian Li and Xianghua Yang of China University of Geosciences, and Fengling Chen, Shiwan Zhang and Zhixiong Fang of the Jianghan Oil Field Inc. of SINOPEC for discussions and sug-gestions. Special thanks also go to AAPG Scientist Jingyao Gong and John Shelton for helpful suggestions and comments. Finally, we thank Wanzhong Shi, Hongtao Zhu, Xiaoqiang Hu and Xinjun Chen for their work on this project. REFERENCES CITED Abels, H. A., Aziz, A., Calvo, J. P., et al., 2009. Shallow

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