Oil geochemical and accumulation characterization in the A sag and surrounding oil-bearing structures of the Weixinan depression, Beibuwan Basin

Based on geochemical characteristics analysis of crude oil samples from different typical wells in the Weixinan sag A and its surrounding areas in the Beibuwan Basin, the crude oils in this area can be divided into three types (Ⅰ, Ⅱ, and Ⅲ). The first type of crude oil (Ⅰ) mainly distributed in the shallow reservoirs has relatively heavier oil density, relatively lighter isotopic value, relatively lower abundance of 4-methyl sterane and lower thermal maturity. The third type of crude oil (Ⅲ) mainly distributed in the deep reservoirs has relatively lower oil density, relatively heavier carbon isotopic value, relatively higher abundance of 4-methyl sterane and the highest thermal maturity. The overall geochemical characteristics of second type of crude oils (Ⅱ) mainly distributed in the shallow reservoirs are in between. Most of the crude oil of the Weixinan sag A belong to class Ⅱ and class Ⅲ. The comparison of geochemical characteristics of source rocks samples shows that there are three sets of source rocks developing in the Weixinan Sag. The upper part source rocks that deposited in an oxidizing environment are dominated by algal input with certain contribution from terrigenous higher plants. The sedimentary environment of the middle part source rocks was more oxidized, and higher plants contributed more to its source materials. The lower part source rocks were formed in a weak reducing-oxidizing environment with large amounts of algal inputs. Oil-source correlation results show that type Ⅰ and type Ⅱ oils are mainly derived from the upper part source rocks, and type Ⅲ oils are chiefly derived from the lower part source rocks. Considering the comprehensive analysis of generation stages, accumulation stages, and migration pathways, this study concludes that type Ⅰ oil, which mainly derived from the first hydrocarbon charging stage of the upper part source rocks, is accumulated in the overlying reservoir by migrating upward through sand bodies. Type Ⅱ oil came from the second hydrocarbon charging stage from the upper part source rocks and accumulated in traps in shallow reservoirs on the periphery of the A Sag. Type Ⅲ oil was generated from the inner center of sag A and migrated along structural crests through longer migration pathways, and finally accumulated into oil reservoirs when being stopped by big faults and faults between formations. The results of this study not only are helpful to further understand the hydrocarbon accumulation and distribution, but is also significant towards the further breakthrough of petroleum exploration in sag A.