Effect of Salinity on Source Rock Formation and Its Control on the Oil Content in Shales in the Hetaoyuan Formation from the Biyang Depression, Nanxiang Basin, Central China

Abstract

A total of 75 shale samples from the Biyang Depression were analyzed with Rock-Eval, total organic carbon (TOC), organic maceral, and gas chromatography–mass spectrometry (GC–MS) techniques to reveal the effect of salinity on source rock formation and its control on the oil content. On the basis of the diversities in salinity and redox conditions reflected by the gammacerane/αβ C30 hopane (G/H) ratio, the pristine/phytane (Pr/Ph) ratio, and the extended tricyclic terpane ratio [ETR = (C28 + C29) tricyclic terpane/Ts)], three types (T1, T2, and T3) of shales were identified, which deposited in the brackish condition (G/H ratio < 0.3), the semi-saline condition (G/H ratio = 0.3–0.6), and the saline condition (G/H ratio > 0.6), respectively. The comparisons among T1, T2, and T3 shales revealed that the salinity had a significant effect on the paleoproductivity of the lacustrine system. Most algae thrive in semi-saline conditions (T2) but are restrained in saline conditions (T3), so that moderate salinity conditions (G/H ratio = 0.3–0.6) are most conducive to the accumulation of algae organic matter (AOM). Although they have similar thermal maturity (0.6–1.1% vitrinite reflectance) and kerogen type (type II), the T1, T2, and T3 shales were developed in different reducing environments indicated by the Pr/Ph ratio and contained variable abundance of organic matter, resulting in remarkable differences in the preservation condition of AOM and the shale oil content within the T1, T2, and T3 shales. The classification and evaluation criteria of the shale oil resource were performed according to three categories based on the oil content (S1) and TOC values. It was revealed that the T1, T2, and T3 source rocks were mainly distributed among these areas of the potential, enriched, and ineffective shale oil resources, respectively, suggesting that shales developed in semi-saline environments deserve to be the most desirable target of shale oil exploration and development at present. Thus, this work may help related industry practitioners acquire valuable information from lacustrine shale systems.

Influence of a Paleosedimentary Environment on Shale Oil Enrichment: A Case Study on the Shahejie Formation of Raoyang Sag, Bohai Bay Basin, China

“…The TOC content first increased and then decreased with increases in paleosalinity (Figure 7F). TOC was more enriched when the value of Sr/Ba was 1.0-3.0, indicating that both low paleosalinity and excessive paleosalinity were not conducive to the enrichment of OM, which is consistent with the results of He et al (2018); based on their use of biomarker compounds in paleosalinity analysis.…”

Characteristics of Shale Wettability by Contact Angle and Its Influencing Factors: A Case Study in Songliao

“…Wettability is important for selecting the appropriate fracturing fluid to minimize fracturing fluid losses. It is generally accepted that the highest crude oil recovery is achieved under low oil-water interfacial tension and weak water-wetting conditions during water displacement (He et al, 2018;Li et al, 2021;He et al, 2022).…”

Physical simulation of residual oil displacement production in offshore strong bottom water reservoir

“…First, the magnitude of the increase in gas injection rate and the technical and economic feasibility is demonstrated according to the reservoir's geological characteristics and fluid properties. Good scheme design and injection production parameter optimization are carried out (Li et al 2007;He et al 2018;Mahani et al 2017). For the strong bottom water reservoir in the middle and late stage of offshore development, it is challenging to implement technologies such as perfect injection production pattern and steam displacement, which are mainly controlled by the excellent production system, perforation location optimization, and gas injection or gas foam mining technology based on existing well pattern.…”