Pore Type and Pore Size Distribution of Tight Reservoir in Permian Lucaogou Formation of Jimsar Sag, Junggar Basin, NW China

Abstract

The Jimsar Sag, located in southeast of the Junggar Basin, is well known for its commercial production of tight oil in China, with a geological oil resource of 2 billion tons. When compared with the Marine shale reservoirs in the Bakken Formation, the lacustrine tight reservoirs in Lucaogou Formation are much more complex. In this study, X-ray diffraction (XRD), total organic carbon (TOC), optical microscope, and scanning electron microscope (SEM) were used in the qualitative analyses of the characteristics of petrography and pore microstructures, as well as in the quantitative analyses of the pore size distribution. Here, we assume that pore size follows a power law distribution. The results show that the major rock types are dolomite and siltstone, which contain different assemblages of pore type with various pore size distribution patterns. Based on the pore origin, three categories and six subcategories were identified: primary mineral pore, dissolved mineral pore, organic matter modified mineral pore, primary organic matter pore, secondary organic matter pore and crack. In dolomite, the entire pore network mainly consists of primary mineral pore. The decrease in dolomite content and the increase in dolomite particle size are commonly in accordance with the decrease of power law exponent D, which denotes the increase in the proportion of the large pore. Compaction and cementation are the principal diagenesis that have a crucial effect on the pore size distribution with the largest exponent D = 2.77. However, in siltstone, secondary dissolved mineral pore, organic matter modified pore and intraparticle pore within authigenic minerals are the predominant pore types, in which case dissolution and cementation were compensating their influence on the pore size distribution with a fluctuating exponent D ranging from 1.5 to 2.7. Thus, the power law exponent D is associated with lithology, diagenesis as well as hydrocarbon generation, which can be used to evaluate tight reservoir and advance our ability to exploit producible zones - “sweet spots” more accurately.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018