Characteristics and Genetic Mechanism of Pore Throat Structure of Shale Oil Reservoir in Saline Lake—A Case Study of Shale Oil of the Lucaogou Formation in Jimsar Sag, Junggar Basin

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.

Three-Water Differential Parallel Conductivity Saturation Model of Low-Permeability Tight Oil and Gas Reservoirs

“…The electrical conductivity of low-permeability tight sandstone rock reservoirs (C t-tr ) is derived from the parallel combination of the resistivity of the strongly bound water, weakly bound water, and free water, as depicted in Equation ( 6). By utilizing Archie's equation (Equations ( 7) and ( 8)) to express the electrical conductivity of the strongly bound water, weakly bound water, and free water in Equation ( 6) and assigning the corresponding cementation and saturation indices to the three types of pores, we derive the comprehensive enhancement of the parallel electrical conductivity of lowpermeability tight sandstone rock reservoirs with varying formation water, as illustrated in Equation (9). Further exploration of the formation conductivity equation under complete saturation conditions, where the saturation of the three types of pores in Equation ( 9) is 100%, results in the precise expression of the electrical conductivity of low-permeability tight sandstone rock reservoirs under full saturation, as depicted in Equation (10).…”

“…The complex geological processes in low-permeability tight sandstone reservoirs have resulted in a complex pore structure characterized by the disappearance of primary pores and the prevalence of dissolution pores. As a result of this complex pore structure, traditional saturation models are ineffective for evaluating logging saturation in low-permeability tight sandstone reservoirs [7][8][9][10][11]. The saturation model based on the rock physics volume model utilizes the theory of multiple connected pores for electrical conduction, essentially taking into account the electrical conductivity of water in the formation.…”

Exploration of Oil/Water/Gas Occurrence State in Shale Reservoir by Molecular Dynamics Simulation

“…Among them, shale reservoirs with micro-nanopores have received great attention, and their position in the energy pattern is becoming increasingly prominent [6]. However, due to the widely distributed nanopores with different types and pore sizes [7][8][9], shale reservoirs generally have low permeability, small porosity, and complex fluid occurrence states [10,11]. Therefore, studying their fluid behavior has important theoretical and practical value.…”

Microscopic Conductivity Mechanism and Saturation Evaluation of Tight Sandstone Reservoirs: A Case Study from Bonan Oilfield, China

“…In the simulation process, a three-dimensional digital rock model needs to be constructed. However, due to the different scales of pores and throats, it is difficult to build a real three-dimensional digital rock of tight sandstone [32]. There are two kinds of digital rock reconstruction methods, numerical reconstruction method, and laboratory experimental method [33].…”