Geochemical characteristics and oil accumulation significance of the high quality saline lacustrine source rocks in the western Qaidam Basin,NW China
Zhang Bin;He Yuanyuan;Chen Yan;Meng Qingyang;Yuan Li;PetroChina Research Institute of Petroleum Exploration and Development;Research Institute of Exploration and Development,PetroChina Qinghai Oilfield Company;
It is traditionally believed that the saline lacustrine source rocks in western Qaidam Basin have low organic matter abundance,and the hydrocarbons are mainly sourced from "soluble organic matters".In this study,a certain scale of high quality saline lacustrine source rocks with high organic matter abundance has been discovered for the first time;the TOC value is about 1% with the maximum above 4%,the potential of hydrocarbon generation is generally greater than6 mg/g with the maximum up to 40 mg/g,and the hydrogen index is commonly above 500 mg/g(TOC)with the maximum above 900 mg/g(TOC).The organic matters of Type I and Type II1 are dominant,mixed with a small amount of Type II2,indicating,the oil-prone types.The samples with ultra-low organic matter abundance(TOC0.5%)indicates the poor types,contain a less content of soluble organic matter and a small potential of hydrocarbon generation,belonging to the inactive source rocks.Among the biomarkes of high quality source rocks,the compounds rich in phytane,C28-sterane,gammacerane and C35-hopance revealing the saline environment;the characteristics of high sterane low hopane,high C27-sterane and low C29-sterane,indicating organic matters are mainly derived from hydrobiontic algae.As socalled oil algae,these algae rich in lipid compounds can be directly converted into hydrocarbons in the low mature stage.The saline environment is conductive to the preservation of liquid hydrocarbon directly converted by algae,thus forming"immature-low mature oil".The content of"soluble organic matter"contained in the saline lacustrine source rocks of western Qaidam Basin is significantly higher than that of fresh water lacustrine source rocks,which is the important material source for a batch of immature-low mature oilfields discovered in regional middle-shallow layers in western Qaidam Basin.It is validated by simulation test that organic mattersalso have a high potential of oil-generation in the mature stage,in accordance with the classic oil-gas generation model.Thus,the oil and gas generated in mature source rocks is the major exploration target for deep layers.In this region,there are two types of hydrocarbon accumulation models for high quality source rocks in Paleogene deep layers.One is outer-source accumulation,i.e.,oil and gas migrates towards shallow tectosphere through open faults for accumulation;the other type is inner-source accumulation,i.e.,oil and gas is accumulated near the source in the lithological traps adjacent to deep source rocks.So far,significant breakthroughs have been made in these two types of oil-gas reservoir exploration,and two blocks with the reserves up to a hundred million tons are discovered to confirm the enormous exploration potential of oil-gas reservoirs formed in deep high quality source rocks.
【CateGory Index】： P618.13
【CateGory Index】： P618.13