Natural fractures in metamorphic basement reservoirs in the Liaohe Basin, China

The Archaean metamorphic basement reservoirs that are characterized by the development of natural fractures are the primary target for oil and gas exploration in the Liaohe Basin. Based on the analyses of outcrops, cores, thin sections, imaging logs, and laboratory measurements, tectonic fractures are the dominant type of natural fractures, and their development is mainly influenced by petrology and faulting. Tectonic fractures are more developed in metamorphic rocks with a higher content of brittle minerals, while fracture orientations are primarily parallel or almost parallel to the fault strikes. Opening-mode fractures, which account for more than 65% of the total reservoir porosity, provide the primary storage space for metamorphic basement reservoirs. Besides, opening-mode fractures serve as the major pathways for fluid flow throughout the reservoirs and ultimately determine the quality of these reservoirs. Therefore, the discrepancies in fracture development are speculated to be the primary reason leading to the vertical stratification of reservoirs. According to the degree of fracture development, various rock types in metamorphic basement reservoirs can be arranged in order as leptite, granulite, migmatitic granite, gneiss, lamprophyre, diabase, and amphibolite. In this sequence, the former rocks are more likely to become effective reservoirs, while the latter ones are usually inner interlayers. However, the lithology boundary of reservoir rocks and inner interlayers is not definite in this sequence. In a specific metamorphic basement reservoir, the rocks that can become reservoirs have a lower limit in the sequence. But for numerous metamorphic basement reservoirs in the Liaohe Basin, there is no lower limit for rocks to become hydrocarbon-bearing reservoirs.