Abstract:

Shale oil and gas exploration and development in China has become increasingly important strategically, as breakthroughs are continually to be made with the improvement of unconventional oil and gas exploration theory and technology. The resource potential of shale reservoirs is largely constrained by reservoir quality, where fracture development is key to oil/gas enrichment and reservoir productivity. Fractures have always been a research focus of organic-rich shale oil and gas reservoirs as they play an important role in petroleum storage space and seepage channels. Shale reservoirs in China are naturally fractured in general and have relatively wide distribution of non-tectonic fractures. Presently, tectonic fractures have been studied more in depth. This paper, therefore, focuses on non-tectonic fractures with comprehensive review of recent domestic and foreign research achievements, focusing on fracture classification, identification and characterization as well as fracture controlling factors, stages and evolutionary sequences. Non-tectonic fractures are characterized by complex formation mechanism and morphology, irregular distribution pattern and small scale. According to their formation mechanism, non-tectonic fractures can be divided into four categories: diagenetic fractures, abnormal high-pressure fractures, bedding fractures and supergene fractures; whereas fracture identification and characterization are still largely based on descriptive analysis. Although the main fracture controlling factors differ between different fracture types, they share certain commonalities as all fractures are controlled, to some degrees, by sedimentation, diagenesis, mineral composition/content and rock mechanical properties. This paper further analyzes the key research challenges and points out key development trends in the field. First, a novel method should be developed to perform comprehensive and effective identification and quantitative characterization of non-tectonic fractures of different types and scales, based on typical characteristics of shale reservoir core, thin section, imaging logging and conventional logging, combined with advanced image technology. Second, as non-tectonic fractures exhibit “multi-genetic type, multi-control factor, multi-stage evolution” characteristics, with paleotemperature/paleopressure, paleofluids and paleodiagenesis playing a key role in their generation and evolution, advanced experimental analysis techniques, such as fluid geochemistry, isotope geochemistry (C, O, Sr) and in situ isotope chronology (U-Pb, Sm-Nd) of common filling materials such as calcite should be the key techniques to determine their formation time, active stages and evolutionary sequence. Last, the future trend is a shift from qualitative/semi-quantitative analyses of fracture development under a single controlling factor to quantitative, multi-factor coupling analysis. That is, using mathematical methods to determine the weights of different controlling factors and construct a quantitative relationship model to quantify the relationship between a comprehensive fracture development index and multiple controlling factors.

Key words: shale oil and gas, non-tectonic fracture, fracture classification, main control factor, fracture development degree, fracture formation period