Geologic Features and Resource Potential of Tight Sand Gas in China

Guo Bincheng; Li Xin;Li Jianzhong

0 Introduction

Tight sand gas was found earlier, for example, we found tight sand gas of Zhong ba in the west of Si chuan Basin in 1971, and we also found a lot tight sand gas in some other basins such as Erdos Basin, but it did not get attention and developed slowly for the limit of the engineering level. Since 2005, the exploration and development of the tight sand gas in Si chuan Basin and Erdos Basin achieve important progress along with the breakthrough and application of fracturing technology, especially, the exploration and development of Surig gas field in Erdos Basin entered a stage of rapid development and this gas field became the biggest gas field of china in 2011. By the end of 2011, tight sand gas in china had been an important field of the natural gas reserve.

1 Main types of tight sand gas

Since we found this unconventional gas reservoir, there have been different classification schemes for the types of tight sand gas reservoirs from different scholars. Literature research indicate the division basis of the type of tight sandstone gas reservoirs change gradually from the reservoir physical property, diagenesis, and accumulation period to more simple classification of the tectonic location. The tight sand gas resources of china are widely distributed in the Erdos Basin, Sichuan Basin, deep Bohai Bay area, Qaidam Basin, Songliao Basin, southern Junggar Basin, southwest of Tarim Basin, Kuche area, Chuxiong area and East China Sea basin. The tectonic evolution of the zones of tight sand gas reservoir showed continuous subsidence in some areas also showed repeatedly lifting in some other areas. Based on exploration practice and studies, the tight sand gas reservoirs in china mainly distribute in the downward direction area of basin slope, deep parts of tectonic syncline and anticline region, and few of them distribute in the central basin or in the deeper parts of sag. Based on the statistics and analysis of the feature of 31 tight sand gas fields from Erdos Basin, Sichuan Basin, Turpan-Hami Basin,Tarim Basin, Junggar Basin, and Songliao Basin, this paper divides tight sand gas reservoir into 3 types, slope lithologic type, structural type and sag type, according to the tectonic locations, combined with the accumulation mechanism, evolution law and the characteristics revealed by the current exploration of tight sand gas reservoirs (table1).

                                                                                                                Table1 the divisions plan of tight sand gas in china

3 Basic Geologic features of tight sand gas

There have the basic geological conditions to develope large tight sandstone gas field from the view of the material foundation of tight sand gas reservoirs in China, the causes and characteristics of tight reservoir, the relationship between hydrocarbon source rock and reservoir and the features of resvoir. Overall, the features are that coal hydrocarbon source rocks and tight resvoirs distribute widely and are closely adjacent, gas fill reservoir in three-dimensional mode, local enrichment emerge.

3.1 The material foundation of the formation of tight sand gas reservoirs is that coal source rocks fill reservoir constantly.

In all type of source rocks, coal bed are highly rich in organic matter, and its features are the content organic matter and dissoluble organic matter is high, but transformation ratio is low, ability to produce gas is strong, which is determined by the humic organic matter. Coal bed distribute widely in sedimental basin of china, and according to the features and evolution of coal accumulation process, in china, Coal accumulation period can be segmented into the late Paleozoic, Mesozoic and Cenozoic. Coal source rock in these periods is good gas source rock to formate tight sand gas reservoirs because abundance of organic matter is high.

Now, tight sand gas field is found in main oil and gas bearing basin which formated in these three periods, such as Sulige in Erdos Basin, Xujiahe in Sichuan Basin and Kuche big gas region in Kuche Basin. Thus, the features of coal source rock in china are wide distribution, high hydrocarbon generation intensity, and generation widely and constantly.

3.2 Tight reservoir is the basic feature of tight sand gas reservoir, which is controlled by diagenesis.

The basic feature of tight sand gas reservoir is tigh reservoir and poor physical property. The ratio of the samples from Sulige in Erdos Basin and Xujiahe in Sichuan Basin, those porosity is from 4 percent to 10 percent and those recompression permeability is less than

0.1mD, accunt for from 80 percent to 92 percent. The tight sand gas reservoir in china is mostly related to development of coal bed. The reasons which coal bed becomes low permeability reservoir are that the deposition and diagenesis environment is acid, and tectonic background is compaction. Low permeability reservoir is mainly influenced by diagenesis, including Compaction, cementation, metasomatism, denudation and clay mineral transformation. Different diagenesis have different influence, and main diagenesis are compaction ( including mechanical and chemical compaction) and cementation.

3.3 Source and resvoir are closely adjacent, and gas move nearly and vertically

Tight sand gas have the features that source and resvoir are closely adjacent, and gas move nearly and vertically, according to study on forming mechanism and exploration practice. The upper Paleozoic beds in Erdos Basin form good reservoir cap assemblage because inheritance river delta sedimentary system develop, sand body distribute widely, multi-layer sand overlap, sand and coal source rock interbed and are closely adjacent.

Tight sand gas resvoir in upper Triassic series formation Xujiahe section in Sichuan Basin show that sand and source rock are closely adjacent, source-reservoir-cap looks like a sandwich. Reservoir in Xu 2、4、6 sections direct contact with the source rock in Xu 1、3、5 sections. Gas from the underlying source rocks can move vertically into reservoir in Xu 2、4、 6 sections, also can move by the fracture of the source rock. Wide generation hydrocarbon combined with a large area of reservoir sand body, then hydrocarbon built up pressure, eventually, gas move nearly and vertically into reservoir drived by pressure difference.

3.4 Bearing gas in a large area in the style of stereoscopic, local enrichment.

Source rock, reservoir and cap contact closely, which form a "sandwich" structure overlapped geological conditions, greatly increasing expulsion efficiency and discharge residue of hydrocarbon from source rock to reservoir, and the expulsion efficiency is high enough to form a large area of accumulation. Such as Ordos basin, the structure is smooth and stable, the generation and expulsion of highly mature coal measures source rocks distributed widely, and the source rock stack with a large area of low porosity and perm eability unconventional tight sandstone reservoirs, forming a large area of "three-dimensional" gas-bearing zone.

The tight sand gas bear gas in a large area, but the extent of enrichment is controlled by favorable facies, high quality reservoir, fracture development, and local construction, showing the characteristics of local enrichment. For example , the Single-layer thickness of Xu 2、4、6 sections in the middle of SiChuan basin is 3 to 8 meters, cumulative thickness is 20 to 50 meters, so thickness vary strongly, but the sand body superimposed longitudinally , stacked contiguous laterally, and contain gas in a large area. The gas reservoir in Xujiahe distribute in a wide range, but concentrate locally. Research and exploration practice confirmed that the tight sand gas reservoir of Xujiahe section in the middle of SiChuan basin mainly distribute in delta frontal zone, this zone is characterized by the development of stacked distributary channel sand body. The sedimentary facies macro-control the distribution of gas reservoirs. The distributaries’ channel and the channel mouth bar facies is the favorable zones for natural gas concentration.

4 Resource potential and development prospects of tight sand gas in china

The analogy evaluation results of tight sand gas show that tight sand gas is rich in major onshore basins in China, and the ultimate resource of tight sand gas is 8 to 11 × 1012 m3, with a big development potential. Based on the resource and exploration and development in the current status, it is expected that tight sand gas will become an important part in the development of the gas industry in China.

5 Conclusions

The tight sand gas reservoir is divided into 3 types, slope lithologic type, structural type and sag type, in China. All of them can form gas in large areas. Basic geological features of tight sand gas reservoirs show that mainly in coal source rock, hydrocarbon generate with a high strength, and constantly fill the reservoir; Tight sand and hydrocarbon source rocks in close proximity to a large area of contact, and hydrocarbon mainly move nearly and vertically into reservoir；Source and reservoir allocate efficiently, bearing a large area of gas in the style of stereoscopic, but local enrichment. The tight sand gas is rich in China, and the exploration and development technologies of tight sand gas has become more mature, so we have conditions to accelerate the development. Tight sand gas is bound to play an important role in natural gas development of China in the future.

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AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013