Characteristics And Formation Environment Of Carboniferous Source Rocks In The Santanghu Basin

The Carboniferous source rocks in the Santanghu Basin are mainly developed in the Upper Carboniferous Kalagang Formation and Haerjiawu Formation.The distribution of Carboniferous source rocks is very important for the formation and exploration of Carboniferous oil and gas reservoirs.On the basis of core description and thin section identification,this paper identifies and corrects the lithology of Carboniferous source rocks,re-statistics the thickness of Carboniferous single-well source rocks,clarifies the vertical development characteristics of source rocks,and uses the method of well-seismic combination,summarize the seismic reflection characteristics of volcanic sedimentary lithofacies and trace them to determine the lateral distribution of source rocks.By carrying out a series of geochemical experiments such as whole rock X-ray diffraction,TOC measurement,rock pyrolysis,organic matter extraction and separation,kerogen micro-component and its carbon isotope,Ro determination,saturated hydrocarbon gas chromatography,etc.The source rock characteristics of the interval are analyzed,the source rock formation environment is analyzed,and the accumulation effect of different source rocks is analyzed based on the oil-source correlation.The research shows that the three sets of source rocks developed in the Upper Carboniferous Kalagang Formation and the Haerjiawu Formation in the Santanghu Basin are all effective source rocks,and the source rocks of the Kalagang Formation have determined three lithology,which are tuffaceous mudstone,dark gray mudstone and dolomitic mudstone;The source rocks of the Haerjiawu Formation have identified two lithology,which are tuffaceous mudstone and carbonaceous mudstone,among which tuffaceous mudstone is concentrated in the upper member of the Haerjiawu Formation,carbonaceous mudstones are concentrated in the lower member.Vertically,the source rocks of the Kalagang Formation have good continuity,and the cumulative thickness is between 300 and 400 m;the source rock of the Haerjiawu Formation is less continuous than the Kalagang Formation,with a cumulative thickness of about 100 m.Laterally,the source rocks of the Kalagang Formation is limited,concentrated in the Ma 71-Ma 39 well area;and the source rocks of the Haerjiawu Formation is widely distributed,with multiple thickness centers in the upper and lower sections.The carbon content of the source rocks in the Kalagang Formation is concentrated at 0.5%～4%,and the abundance of tuffaceous mudstone is relatively high among the three lithologies;The organic matter abundance of the source rocks of the Haerjiawu Formation is higher than that of the Kalagang Formation,and the organic carbon content is between 2%～20%.The organic matter abundance of the tuffaceous mudstone is lower than that of the carbonaceous mudstone,and the organic matter abundance in the upper part is slightly lower than the organic matter abundance in the lower section.The parent material types of the Kalagang Formation source rocks and the Haerjiawu Formation source rocks are mainly type II.The source rocks of the Haerjiawu Formation have a moderate degree of evolution,and the whole is in the mature oil production stage.Hydrocarbon generation ability is very strong;The source rocks of the Kalagang Formation have a relatively low degree of evolution and are generally in the low-mature to immature stage,and their hydrocarbongenerating capacity is significantly lower than that of the Haerjiawu Formation.The oilsource comparison results show that the Carboniferous crude oil can be divided into three categories as a whole,all of which come from the source rocks of the Upper Carboniferous Haerjiawu Formation and the Kalagang Formation,and the distribution of source rocks has obvious control over oil and gas reservoirs.The sedimentary environment of the source rocks of the Haerjiawu Formation is generally continental,which can be further divided into two environments: limnology and alluvial plains,and the source rocks are mainly developed in the limnological environment;The source rocks of the Kalagang Formation are deposited in a lacustrine environment.Therefore,the main source rock of the Carboniferous system in the Santanghu Basin is the Haerjiawu Formation.