Characteristics and Model of Heterogeneous Wettability in a Marine-Continental Transitional Shale Reservoir: Insights from Organic Matter, Minerals, and Microstructure

Abstract

Heterogeneous wettability is characterized by a spotted-wet or mixed-wet state. It has a great influence on reservoir evaluation and the efficient development of shale gas reservoirs. In this study, reservoir properties of several core plugs from the Permian Shanxi–Taiyuan formations were studied. These formations are typical marine-continental transitional shale located in the Southern North China Basin. To investigate the effects of reservoir properties on heterogeneous wettability, we measured the water–air contact angle and compared it with other properties such as organic petrology, organic geochemistry, mineralogy, and microstructure features, including pore-fractures and surface roughness. The results reveal a negative correlation of vitrinite content with contact angle, in addition to the high clay content and residual polar groups within type III kerogen, indicating that the Shanxi–Taiyuan shales with a high maturity level still have a higher affinity to water. The contact angle of the core samples decreases with increasing surface roughness, partially due to the influence of pore-fracture development. The crossplots indicate that the majority of pore-fractures that exist in the shale preferentially tend to be water-wet. Therefore, the heterogeneous wettability of shale is dominated by the random mixture and arrangement of hydrophilic and hydrophobic components as well as the complexity of the microstructure, such as the rough pore wall. Furthermore, on the basis of improving and examining the Cassie model, a triangle method and a detailed workflow for evaluating the heterogeneous wettability of shale are proposed by comprehensively analyzing “three factors”, including pore structure, mineral components, and organic matter. The test results demonstrate that the Shanxi–Taiyuan shales are mainly water-wet (controlled by organic factor) and neutral-wet, which evidently differ from the marine Longmaxi shale with a wide distribution covering the zones of strongly water-wet, weakly water-wet (controlled by pore factor), and weakly water-wet (neutral-wet). The proposed approach can be applied to promptly and comprehensively evaluate and predict the heterogeneous wettability of shales during shale oil and gas exploration.

Siliceous Origin of the Cambrian Qiongzhusi Formation Shale in the Middle Part of the Upper Yangtze Platform: Significance of Organic Matter Enrichment

“…The basic framework is composed of relatively stable Yangtze land blocks and relatively active South China active zones. During the period from the end of Ediacaran to the early Cambrian, the Yangtze plate and the Cathaysia plate were in a strong tension stage and received many sediment fragments in the early Cambrian, forming a sedimentary sequence of clastic rock, phosphate rock, and black shale. ,, Afterward, the Yangtze Plate underwent a transition from tension to thermal subsidence, and from the Early Cambrian to the Early Ordovician, Li Wei and others limited the time limit of the “Xingkai Movement” to the Early Cambrian, believing that it could be divided into two episodes: I and II, which occurred in the early sedimentary stages of the Maidiping Formation and Qiongzhusi Formation (Niutitang Formation) of the Early Cambrian. The first episode of Xingkai Movement mainly formed the tension trough or Aulacogen between Gaoshiti-Moxi and Weiyuan-Ziyang in Sichuan Basin.…”