1 TIGHT SANDSTONE GAS RESERVOIRS (EXPLORATION) IN UPPER PALEOZOIC OF ORDOS BASIN YANG Hua FU JinHua WEI XinShan REN JunFeng (PetroChina Changqing Oilfield Company, Xi’an, 710018, China) Key words Ordos basin Upper Paleozoic, Tight sand, Reservoir 1 Introduction Over the past decade, in the Upper Paleozoic of Ordos basin, PetroChina Changqing Oilfield Company has been eye-catching for a large 100-billion-cubic- meters-level gasfield found every two years averagely. Especially in recent years, a situation of progressive incremental of 500 billion cubic meters natural gas reserves submitted annually has been formed in the Sulige exploration area. The rapid growth of proven gas reserves makes Sulige region become the first unitary large-scale natural gas district with trillion cubic meters in China [1-4] . The annual gas production from tight sandstone reservoirs reaches more than 10 billion cubic meters. For tight sandstone gas reservoirs in Ordos basin, a number of scholars have been carried out a lot of discussion in the relevant literatures [5-34] . But most of them concentrated in the discussion of common features such as low porosity, low permeability, low abundance, low-pressure, no bottom-water, no edge-water etc., and less attention to the characteristics of their personality. At present, nearly 1000 wells are drilled through the Upper Paleozoic of the basin. Yulin, Zizhou and Sulige gas fields have been put into large-scale development one after another. In-depth exploration and development of Upper Palaeozoic tight sandstone gas reservoirs provides favorable conditions for revealing the characteristics and controlling factors of accumulation. Based on natural gas exploration and development practice and comprehensive geological study results of the basin’s Upper Paleozoic in recent years, the authors mainly discuss the geological features and unique reservoir controlling factors of this type of gas reservoirs in the paper, looking forward to improve constantly the geological theory of tight sandstone gas accumulation and to promote continuous deepening of exploring for such gas reservoirs. 2, Geological background Located in the middle part of China’s mainland, the Ordos basin is the country's second largest sedimentary basin covering a total area of 37 × 10 4 Figure.1 Location of the Ordos basin, structural units and cross section A B A B
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TIGHT SANDSTONE GAS RESERVOIRS (EXPLORATION) IN UPPER PALEOZOIC OF
large-area distribution and reservoir sand bodies of
multi-type genesis.
And the large-area distribution of sand bodies has
been confirmed by drilling. Taking the He8 Member
of Lower Shihezi Formation as an example, based
on statistics from 234 exploratory wells, the
probability of penetration of sandbodies is 92%, and
probability of penetration of gas reservoirs is 65%.
Probability of penetration of sandbodies in Sulige
area is more than 95%, and so is the probability of
penetration of gas reservoirs.
4.3.2 High-energy facies belts and constructive
diagenetic facies co-act to form "dessert" of relative
hypertonic districts
Figure.9 Sedimentary model of shallow delta
in upper Paleozoic
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Comprehensive geologic study and analysis show
that, during the fluvial - delta deposition, rock
composition of different sedimentary facies belts has
significant differences, because of differences in
hydrodynamic conditions. And it further affects the
late diagenetic evolution and characteristics of
porosity development. A corresponding relationship
can be established between them. The main
distributary channel sand body was mainly
composed of mid-coarse-grained quartz sandstone,
with the strongest hydrodynamic conditions, and
good sorting of rock, high content of rigid particles of
quartz, and increased anti-compaction. This was
conducive to preservation of primary porosity, pore
fluid activities and the formation of secondary pores.
The dissolution diagenetic facies are mainly formed,
with main pore type of intergranular and dissolution
pores, and a good capability of reservoir. It generally
can form a local "dessert" of relative high
permeability district. Due to the weakening of
hydrodynamic conditions, the edge parts of the two
channels and the natural levee sites take generally
fine-grains as the dominant, with sorting deteriorated,
and increased content of soft components like debris.
In the course of the late diagenetic evolution, the
reservoirs are easily compacted and become dense,
with primary pores compacted out basically. Further
more, the tight reservoirs are not conducive to the
pore fluid activities and the formation of secondary
dissolution porosity. Diagenetic facies take alteration
and compaction as the dominant. Reservoir
properties were gradually deteriorated.
4.4 Excellent preservation conditions
4.4.1 Upper Shihezi Formation regional caprock
with good sealing conditions
The lacustrine mudstone of Upper Shihezi
Formation developed above the major gas layers of
Upper Paleozoic was the regional caprock of Upper
Paleozoic. This set of mudstone is distributed
continuously in the whole basin, with thickness of 80
m–110 m. The average permeability of mudstone or
sandy mudstone is between 10-10
and 10-8µm
2, the
breakthrough pressure is higher than 5 MPa, so the
regional cap rock provides good sealing condition for
conservation of large-scale lithologic gas reservoir.
4.4.2 The stability of Cratonic block guarantees
the late preservation of gas reservoirs
Destruction in the late stage was the prominent
problem of continental facies oil and gas reservoirs
in China [55-59]
. However, the stability of craton
during late evolution on Yi-Shaan slope of Ordos
Basin avoids such a problem effectively, which is
featured by weak magmation and steady structure
and dominated by overall uplift and subsidence.
Therefore, the present monocline structure with flat
formation and strata dip lower than 1° was formed,
providing good conservation condition for
large-scale lithologic gas reservoirs.
5 Conclusions
Upper Paleozoic of Ordos Basin develops
large-scale, multi-series strata distribution of the
tight sandstone gas reservoirs, with the
characteristics of continuity. Exploration and
geological research show that the control factors of
the Upper Paleozoic formation of tight sandstone
gas reservoirs include: large-scale distribution of the
coal-bearing source rocks in large areas of
hydrocarbon generation, and vertical migration and
accumulation: large-scale sandbody of
shallow-water delta deposits provided favorable
natural gas gathering places for tight sandstone gas
reservoirs; hydrocarbon generating intensity
Figure.10 Sedimentary evolution in later Paleozoic
in Ordos basin
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controlled the enrichment of the tight gas reservoir; a
good seal of regional cap, and reservoir densification,
as well as the stability of cratonic basin ensured the
preservation of gas reservoir.
The exploration and development of tight
sandstone gas reservoirs are difficult, but after a
long-term exploration and development, Changqing
Oilfield Company has formed a set of tight
sandstone gas reservoirs exploration technology
and methods in Upper Paleozoic of Ordos Basin,
mainly including high-resolution the rate of
two-dimensional seismic prestack sandbody
prediction, seismic prestack gas detection
technology, logging of lithology recognition
technology as well as the multi-line multi-layered
fracturing technology for tight reservoirs. For this
type of gas reservoirs exploration, we should use "
the overall study, development, and evaluation, as
well as the implementation of step-by-step" to
construct Ordos Basin into a base of tight sandstone
gas reservoir exploration and development.
Acknowledgements:
We show our sincere thanks to Mr. LIU XinShe,
ZHAO HuiTao, ZHAO TaiPing and Ms. WANG Xin,
ZHAN Sha, for their helpful materials and graphics.
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