Identification of oil resource and analysis of hydrocarbon migration and accumulation of Majiang paleo-reservoir: Evidence from oil-gas geochemistry

Abstract：This paper takes full account of the interrelation and variation rules of source rock kerogen, group composition, solid bitumen and N-alkane carbon isotope in thermal evolution. Combining the geochemical analysis with previous studies of carbon isotope and aromatic markers, the authors made a comprehensive analysis. It is indicated that, at the early stage, there existed the source of Lower Cambrian black shale in various paleo-reservoirs, whereas at the late stage, the sources became quite different. The bitumens from the south of Majiang ancient anticline were relevant to Middle Cambrian Duliujiang Formation to some extent. The crude oil and oil seepage in Kaili seemed to be the products of Indosinian reservoir accumulation from Wufeng-Longmaxi Formation. The preservation environment of the oil was an independent and closed system formed in the early diagenetic process of the reservoir. Based on oil resource identification in combination with geological background, hydrocarbon accumulation conditions and fluid activity patterns, it is held that the distribution of source rocks in the depression of south Guizhou Province and its neighboring Lower Cambrian Dujiangliu Formation and Wufeng-Longmaxi Formation controlled the hydrocarbon enrichment patterns of this area. The fault system in Hercynian period of south Guizhou and the unconformity formed by Caledonian movement formed the pathways of hydrocarbon migration and accumulation in different periods. A series of normal faults formed in Hercynian constituted the key to Paleo-reservoir formation in Majiang.