Hydrocarbon Analysis About Marine Mesozoic-Palaeozoic In Subei Basin

Subei Basin, part of Lower Yangtze region as well as the onshore part of South Yellow SeaBasin, developed a series of widely distributed and thick marine carbonate rocks. Since theMesozoic and Paleozoic strata are deeply buried and strongly influenced by several tectonicmotions in this region, a multi-stage pattern of overlay superimposed basins is formed. It directlycaused the favorable conditions of reservoirs unclear. The paper mainly analyzedhydrocarbon-bearing characteristics of Mesozoic and Paleozoic marine formation in Subei Basinbased on geological data, geochemical data and other basic information. It mainly talked abouthydrocarbon source rock, reservoir, cap rock, thermal history analysis and the relationshipbetween structure and reservoir.The study collected four sets of source rocks outcrop samples, including samples of Mufumountain group, Gaojiabian group, Permian and Triassic Qinglong group. We analyzed141dataof four indicators including the organic carbon content, maturity index Ro, chloroform bitumen"A" and hydrocarbon generation potential to figure out the regular of hydrocarbon source rocks.Meanwhile maturity index Ro was studied systematically to draw contours of the upper and lowerPaleozoic respectively. There are70outcrop data measured by experiments,23core data and48collated data from research papers on the area. The reservoirs of the studied area are mainly ofcarbonate rock and mostly concentrated in Sinian, Cambrian, Ordovician, Carboniferous, Permianand Triassic strata, other type of clastic reservoirs are mainly concentrated in the DevonianWutong group. By analyzing porosity and permeability data of31outcrop data, combined withmicroscopic observation and lithofacies paleogeography information, reservoir characters aresummarized from three different aspects of diagenesis, reservoir space and spatial characteristics.The relationship between structure and hydrocarbon structure of lower Yangtze basin and tectonicevolution was also discussed. We achieved the following results from the research:1. There are many hydrocarbon centers in the marine strata of Subei Basin. The lithologyof hydrocarbon source rocks in Mesozoic and Paleozoic era are mainly shale and carbonate, theorganic maturity type is mainly of Ⅱ-Ⅲ type.2. Primary porosity of carbonate reservoirs is very low in the study area because ofoccurred compaction, cementation, recrystallization during diagenetic epigenetic. But fissures,dissolved pores and secondary cracks improved the reservoir capacity from the original lowporosity and low permeability. Regional seal are mainly developed in Gaojiabian group andLongtan group in the area. 3. The organic maturity of Mesozoic and Paleozoic era in Subei basin has obvious regionalheterogeneity and Ro is between0.47~2.56. The shallow buried and low mature source rock hasthe conditions of secondary hydrocarbon generation and late-stage accumulation.4. The study area are mainly developed on the Yangtze platform in the Paleozoic era, Mostoil and gas are preserved on the passive continental margin of north and south sides. Multiplesedimentary centers were developed during the Mesozoic era. The main depositional andsubsidence centers are in “Anji trough” and secondary sedimentation centers in “Chuxian trough”.Most of them are deep-water carbonate sediments and concentrated in the Permian Longtan groupand Dalong group.5. There are three different types of hydrocarbon reservoirs in the study area: Triassicfractural and vuggy reservoir type, Permian Longtan group self-generating and self-preservingdense sandstone reservoir type and Cretaceous Pukou group anticline reservoir type, and thesecond type is in the area is the focus of the oil and gas exploration and development researchtoday, mainly developed in Huangqiao area.Favorable sedimentary facies in north Jiangsu area has the trend of extension to South YellowSea, so we can speculate that there are hydrocarbon centers exist in the South Yellow Sea. Manywells drilled through Mesozoic and Paleozoic formation have obtained industrial flows in thestudy area, they are also indicates that Mesozoic and Paleozoic formation of South Yellow Sea hasbroad prospects of oil and gas exploration.