Study On Solid Bitumen Biomarkers In Sinian-Lower Paleozoic Of Anyue Gas Field,Sichuan Province

The exploration in the Sinian-lower Paleozoic stratum of Leshan-Longnvsi uplift have lasted for 50 years,the discovery of the Anyue Gas Field after the major breakthrough in high risk exploratory well GS1 in 2011 has very important significance to promote the development of China natural gas industry and ensure national energy security.However,as the oldest marine carbonate gas reservoir,the gas-generating rocks are in high-over matured stage.The previously formed oil reservoirs have been destroyed by thermal cracking and as a consequence,the pyrolysis caused asphalt are widely distributed in the carbonate reservoirs.The complex geological and tectonic evolution process severely restricted the process of oil and gas exploration in Sinian-Cambrian carbonate of Leshan-Longnvsi upliftIn this paper,Sinian-Cambrian formation of Leshan-Longnvsi uplift in the central Sichuan Basin is studied by mineralogy,organic geochemistry and isotope geochemistry.The widely distributed deep solid bitumen and source rock were carried out systematic geological and geochemical studies.The main results achieved are as follows:1.It can be seen that reservoir bitumen from the three formations have similar geochemical characteristics:Normal alkanes in these bitumens have a smooth unimodal distribution of nC₁₄^<sub>33 with common masima at C₂₂2 and C₂₃,the Pr/Ph all values less than 1,high abundance of the C₂₃ tetracyclic;It has a“V”shape distribution of C₂₇-C₂₈-C₂₉ steranes,the relative abundance of regular sterane is C₂₇>C₂₉>C₂₈ steranes.The results show that the reservoir bitumen in each layer has a common source2.Organic geochemistry shows that the organic matter of Dengying formation and Longwangmiao formation in Sinian-Lower Paleozoic derives from low aquatic algae and bacteria.The formation deposited in anoxic-reducing water condition and provides a favorable environment for organic matter preservation.3.Evidence shows that the reservoir bitumen in Dengying formation and Longwangmiao formation both generated from Dengying formation and Qiongzhusi formation and the two sets of source rock are similar in geochemical characteristics.On the whole,source rock of Qiongzhusi formation deposited in a border area than Dengying formation.Besides,the abundance of organic matter is higher than the latter and makes a greater contribution on hydrocarbon accumulation.4.Based on petroleum filling theory,typical maturity parameters such as C₂₁-/C₂₂+?(C₂₁+C₂₂)/(C₂₈+C₂₉)and tricyclic terpane/C₃₀0 hopance were selected to establish the contours of thermal maturity of reservoir bitumen and according this three main directions of oil charging and migrating during geological history were restored:northwest direction,north direction and southwest direction.It is speculated that the main hydrocarbon source kitchen in the study area are located in the west and north of the Gaoshiti-Moxi area.