Geochemistry On Pyrobitumen And Source Rocks In Deep Layer Of Leshan-Longnvsi Area

With the increasing difficulty in oil and gas exploration and development in middle-shallow strata,oil and gas exploration and development in deep strata is becoming more and more concerned.As a representative of oil and gas exploration in deep strata,the Sinian-lower Cambrian in Sichuan Basin has made remarkable achievements in the study of hydrocarbon accumulation.However,the paleo-oil reservoirs have been destroyed by thermal cracking and as a consequence,pyrobitumen were 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 paleo-uplift.In this paper,the Sinian-lower Cambrian of Leshan-Longguizi paleo-uplift was studied by petrology,mineralogy,organic geochemistry,inorganic geochemistry and isotope geochemistry.The widely distributed pyrobitumen and source rocks were carried out with systematic geological and geochemical studies.The main results achieved are as follows:1.Organic geochemistry shows that the organic matter of Qiongzhusi Formation of the lower Cambrian derives from low aquatic organisms,with high organic matter abundance(average TOC content of 1.8%,the maximum TOC content of 7.26%),high-over thermal maturaty and good organic matter quality.2.The shale of Qiongzhusi Formation of the Lower Cambrian deposited in anoxic –reducing water condition.In terms of Qiongzhusi Formation forming,high,paleoproductivity is the direct factor which controls organic matter enrichment.The widely distributed hydrothermal activity promotes the enrichment of organic matter.The anoxic-reducing water condition provides a favorable environment for organic matter preservation.3.Evidence shows that Dengying formation and Qiongzhusi Formation are two sets of main source rocks in study area through pyrobitumen –source rocks correlation and the two sets of source rocks are similar in geochemical characteristics.The pyrobitumen in Dengying Formation and Longwangmiao Formation were generated from the two sets of source rocks.On the whole,source rocks of Qiongzhusi Formation deposited in a broader 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 reservoir geochemistry theory,typical maturity parameters such as MPI2,TeMNR were selected to trace oil filling path of the paleo-reservoir and according that three main directions of oil charging and migrating were determined: northwest direction,north direction and southwest direction.