Pore Structure, Fractal Features, and Oil Content Evaluation of Lacustrine Shale: A Case Study of First Member of Paleogene Shahejie Formation, NE China

Enze Wang,Yanchen Song,Zhuoya Wu,Jianwei Wang,Hanqi Li,Gang Chen,Yue Feng,Keliu Wu

doi:10.1155/2021/2721088

Abstract

The complex and heterogeneous pore structure and oil content of lacustrine shales introduce significant challenges in the exploration of shale petroleum systems. To further expand our geological understanding of lacustrine shales and provide guidance for hydrocarbon exploration, in this study, based on programmed and improved Rock-Eval pyrolysis, total organic content (TOC) measurements, scanning electron microscopy (SEM) observations, and N2 adsorption (NA) experiments, a systematic evaluation of the mineral composition, geochemical features, pore structure, and oil contents of the third member of the Paleogene Shahejie Formation (Es3) shale in the typical lacustrine hydrocarbon enrichment sag (Nanpu Sag, Bohai Bay Basin) is conducted. The fractal theory is selected to quantify the pore system’s heterogeneity; also, the relationships between the pore structure parameters and the oil contents and mineral composition are revealed. Our results reveal the geochemical features of the Es3 shale are excellent, and thus, they can be classified as a set of high-quality source rocks (average TOC of 4.2%, mainly type II kerogen, and in the mature stage). Based on the SEM and NA analyses, the intercrystalline pores dominate the pore system. Moreover, the mesopores contribute the most specific surface area (SA) and pore volume (PV) to the samples (average contribution rates of 74.7 and 75.0%, respectively). The fractal dimensions have relatively strong positive correlations with PV and SA, which means more heterogeneous and complex pore structures provide more storage space and adsorption sites for shale oil accumulation. The clay mineral content is the most important factor which controls the pore structure parameter, and high clay content indicates high SA and PV. According to proposed numerical movable and adsorption hydrocarbon calculation models, the TOC content is the dominant factor controlling the movable and adsorption oil contents, but other factors’ impact also cannot be ignored. Our study not only provides guidance for shale oil exploration in the study area but also broadens our understanding of the oil contents of lacustrine shale.

Highlights

Shale petroleum system exploration has attracted the attention of academic communities all over the world [1,2,3,4]
Based on the previous three component classification scheme and the structure of shale, the Es3 shale can be classified into three types [38]: argillaceous shale (A), mixed shale (MS), and siliceous shale (S)
The X-ray diffraction (XRD) data show that clay minerals and siliceous minerals are the two main types of minerals, accounting for 30 to 52% and 22 to 59%, respectively

Summary

Introduction

Shale petroleum system exploration has attracted the attention of academic communities all over the world [1,2,3,4]. Compared with conventional petroleum systems (conventional sandstone reservoirs), shale petroleum systems are characterized by self-generated and self-retained reservoirs, which means less hydrocarbon loss through petroleum migration and great resource potential [5]. It is important to accurately calculate the moveable and retention oil for resource evaluation and sweet spot prediction [6, 7]. A clear understanding of the geometrical and geological features of the pore system is of great significance to the exploration of shale petroleum systems [10,11,12]

Results

Discussion

Conclusion