Hydrocarbon Enrichment and Main Controlling Factors in Offshore Rift Basins of China: A Case Study in the Beibuwan Basin

Abstract

Beibuwan Basin is an important offshore oil and gas bearing basin of China. However, the distribution of oil and gas in different sags is uneven, and the rule of hydrocarbon accumulation and main controlling factors have not been clearly understood in this basin. Based on abundant seismic, well and analytical data, the differences and main controlling of tectonic evolution, sedimentary filling and source rock quality in sags are analyzed, and combined with the study of geothermal filed, the main controlling factors of differential hydrocarbon generation in sags of the Beibuwan Basin are clarified. On the basis of the research above, the hydrocarbon enrichment of the basin is clarified through the systematic analysis of reservoir-caprock assemblage and accumulation characteristics of different structural units. The results show that there are four NE-trending sag zones in the basin, and under the influence of fault activity, the scale of sags is regularly distributed. The scale of rifts in different sag zones decreases gradually from northwest to southeast and the scale of rifts in the middle of a single sag zone is normally larger than that at two ends. Under the control of the sag-controlling faults activity, paleoproductivity and organic matter preservation conditions, the quality of source rocks in sags of the Beibuwan Basin decreases from northwest to southeast. The hydrocarbon-rich sags in the Beibuwan Basin were formed under the control of source rocks and geothermal field, and the distribution of large-medium scale fields is controlled by the favorable structural traps in the rift strata of inner gentle slope zone. The understanding of the hydrocarbon enrichment and main controlling factors in the Beibuwan Basin also provides a significant guidance for the oil and gas exploration in other rift basins.