Petroleum System Analysis of the Poorly Devoloped Chagan Sag in Yingen-Ejinaqi, Northern China

Abstract

Hydrocarbon exploration is at an early stage in Yingen-Ejinaqi basin that locates in northern China. Chagan sag that lies in the eastern part of the basin has made some important hydrocarbon breakthroughs recently. To advance our understanding of the reservoir forming process, petroleum system of Chagan sag is studied combing geochemical analysis of oil and core samples and basin modeling work. Oil-source rock correlation results from biomarkers and carbon isotopes suggest that oils accumulated in Chagan sag are derived from the lower Cretaceous black mudstones. Most of the analyzed samples have present-day TOC above 0.5%, revealing that they have met the accepted standard of source rocks with fair generative potential. Vitrinite reflectance values show a medium to over maturation stage of organic matter, thus most of the organic matter is likely to have been converted to hydrocarbons. Burial and hydrocarbon generation history is studied by conducting vitrinite reflectance calibrated 1D basin models. Combined with fluid inclusion petrography, fluorescence spectroscopy and microthermometry data, analysis results show that the hydrocarbon was charged immediately after the hydrocarbon generation peak of the source rocks and coincided with the regional rapid subsidence. Permeable sand bodies and fault system served as the main migration pathways. Two typical cross-sections were also selected to simulate the evolution of overpressure and its effects on hydrocarbon migration. 2D modeling results calibrated with measured vitrinite reflectance and permeability data indicate that the overpressure was built up as a result of quick subsidence and sedimentation rate in early Cretaceous. The widespread low-permeability basalts that retarded pressure transmission and fluid flow played an important role in the generation and distribution of overpressure. The build-up overpressure in the source rock intervals reopened the faults and drove the hydrocarbon vertically into the traps that are above the impermeable volcanic layers. During basin-scale tectonic reverse stage in Cenozoic, some reservoirs were destroyed and hydrocarbon accumulated previously in deeper successions migrated upwards and was subjected to biodegradation. These preliminary understanding of source rock property, hydrocarbon generation history, pore pressure evolution, hydrocarbon migration mechanism provide the basis for exploring new hydrocarbon plays in the whole Yingen-Ejinaqi basin.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016