Hydrocarbon accumulation and formation of composite reservoirs in KLI6-I of Laizhou Bay Sag, Bohai Sea, China

摘要

The Laizhou Bay Sag, one of the oil-bearing sags with large exploration potential in the South Bohai Sea of China, contains two sets of high-quality hydrocarbon source rocks, i.e. Member 3 and Member 4 of Shahejie Fm (E(2)s(3) and E(2)s(4)). As an important hydrocarbon accumulation zone in the sag, the KL16-1 low bulge is found to have oil reservoirs in the Neogene Guantao Fm (N(1)g), the Paleogene Dongying Fm (E(3)d), the Paleogene Shahejie Fm (E(2)s), and the Mesozoic buried hill formation. It is characterized by distinct features of composite reservoirs. Nonetheless, its hydrocarbon accumulation process and mechanism are unknown yet. In this paper, the hydrocarbon generation modelling of source rocks, combined with the characteristics of oil-source rock biomarker compounds and fluid inclusions, is used to restore the hydrocarbon accumulation process in the KL16-1 low bulge. Crude oil in all three KL16-1 plays is characterized by low Pr/Ph, low gammacerane, high 4-methyl sterane and high dinosterane. In other words, it is oil of low maturity to maturity. As the biomarker index of the oil is very similar to that of the E(2)s(3) source rocks, it is thought that E(2)s(3) is the primary source rock layer, while E(2)s(4) is the secondary source rock layer. In addition, episodic oil and gas charging happened in two phases under high pressure in the late period (from 5.0 Ma till now).