Abstract

The shale oil reservoir of the Lucaogou Formation in the Jimsar Sag has undergone tectonic movement, regional deposition and complex diagenesis processes. Therefore, various reservoir space types and complex combination patterns of pores have developed, resulting in an intricate pore throat structure. The complex pore throat structure brings great challenges to the classification and evaluation of reservoirs and the efficient development of shale oil. The methods of scanning electron microscopy, high-pressure mercury injection, low-temperature adsorption experiments and thin-slice analysis were used in this study. Mineral, petrology, pore throat structure and evolution process characteristics of the shale oil reservoir were analyzed and discussed qualitatively and quantitatively. Based on these studies, the evolution characteristics and formation mechanisms of different pore throat structures were revealed, and four progressions were made. The reservoir space of the Lucaogou Formation is mainly composed of residual intergranular pores, dissolved pores, intercrystalline pores and fractures. Four types of pore throat structures in the shale oil reservoir of the Lucaogou Formation were quantitatively characterized. Furthermore, the primary pore throat structure was controlled by a sedimentary environment. The pores and throats were reduced and blocked by compaction and cementation, which deteriorates the physical properties of the reservoirs. However, the dissolution of early carbonate, feldspar and tuffaceous minerals and a small amount of carbonate cements by organic acids are the key factors to improve the pore throat structure of the reservoirs. The genetic evolution model of pore throat structures in the shale oil reservoir of the Lucaogou Formation are divided into two types. The large-pore medium-fine throat and medium-pore medium-throat reservoirs are mainly located in the delta front-shallow lake facies and are characterized by the diagenetic assemblage types of weak compaction–weak carbonate cementation–strong dissolution, early medium compaction–medium calcite and dolomite cementation–weak dissolution. The medium-pore fine throats and fine-pore fine throats are mainly developed in shallow lakes and semi-deep lakes. They are characterized by the diagenetic assemblage type of strong compaction–strong calcite cementation–weak dissolution diagenesis. This study provides a comprehensive understanding of the pore throat structure and the genetic mechanism of a complex shale oil reservoir and benefits the exploration and development of shale oil.

Highlights

  • In recent years, the exploration and development of shale oil have developed rapidly inChina, which is currently a research hotspot of unconventional hydrocarbon resources [1,2,3,4].In China, shale oil is mainly developed in Mesozoic and Cenozoic lacustrine shale strata in the continental basins

  • The pore throat radius of type III is mainly distributed in 0.016–0.063 μm

  • The results of mercury intrusion experiments and pore structure types show that the pore structures of different lithology and physical properties are different in characteristics

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Summary

Introduction

In China, shale oil is mainly developed in Mesozoic and Cenozoic lacustrine shale strata in the continental basins It has huge exploration and development potential [5,6,7,8,9]. The previous researches mainly focused on a single layer and discussed the characterization techniques of the pore throat structure of different lithologies in shale oil reservoirs [22,23,24,25,26,27]. These studies deepen the understanding of the pore throat structure in shale oil reservoirs. The microstructure and genetic mechanism between layers have not been compared, and the effects of sedimentation and diagenetic evolution on microscopic pore throat structure have not been explained

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