Characterization and formation mechanisms of fractures and their significance to hydrocarbon accumulation: A case study of Lower Ordovician mid-assemblage Formations in central Ordos Basin, China

Abstract

The lower Ordovician mid-assemblage Formations in the central Ordos Basin of China host prolific gas resources, and most hydrocarbon reserves are stored in naturally-fractured reservoirs. Thus, fracture pathway systems may have a significant impact on reservoir performance. This article focuses on the core- and laboratory-based characterization of fractures. Through the developmental degrees, extended scale, output state and filling characteristics of various types of fractures, the results show that there are three distinct fracture types: 1) nearly vertical fractures, 2) oblique fractures, and 3) horizontal fractures. Based on a systematic study of the characterization of reservoir space, the main geologic setting of natural gas accumulation and the regional tectonic background, type 1 is mainly driven by the tectonic formation mechanism, and type 3 and parts of low-angle fractures in type 2 are induced by the diagenetic formation mechanism. While recovered paleopressure for methane-rich aqueous inclusions trapped in fracture-filling cement indicates that the fracture opening and growth are consistent with gas maturation and charge and such high-angle fractures in type 2 are caused by the compound formation mechanism. The fractures to hydrocarbon accumulation may play a more significant role in improving the quality of reservoir porosity. Furthermore, connected fractures, dissolved pores and cavities together constitute the three-dimensional pore-cave-fracture network pathway systems, with faults serving as the dominant charge pathways of highly pressurized gas in the study area. Our results demonstrate that protracted growth of a pervasive fracture system is not only the consequence of various formation mechanisms but also intrinsic to quasi-continuous accumulation reservoirs.

Keywords

characterization of fracture / formation mechanism / quasi-continuous accumulation / Ordovician mid-assemblage formations / Ordos basin

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