Multi-stage karst characteristics and reservoir control of early diagenetic limestone in Taiyuan Formation, Ordos Basin, China

Abstract

Recently, several wells, such as YT1H and ZT1 in the Ordos Basin, have made new discoveries of natural gas in the Permian Taiyuan Formation limestone, revealing that the limestone of the Taiyuan Formation has good exploration potential. However, there are still problems such as unclear reservoir genesis mechanism and undefined key reservoir formation mode in the Taiyuan Formation limestone, which seriously restricts further gas exploration and deployment in this layer. Therefore, based on the abundant core, thin section and physical property data of Taiyuan Formation, this paper systematically studies the relationship between limestone reservoir development and early exposed karstification, establishing the karst reservoir control model in the early limestone diagenesis. The results show that: (1) Early diagenetic karstification primarily developed in granular limestone and mostly located in the middle and upper parts of the upward-shallowing sequence. Meanwhile, the identifiable karst features include fabric selective dissolution, solution fissures/solution gullies, dissolution speckle, karst breccia, and multi-phase exposed surfaces. (2) Karst strength within a single cycle gradually increases from the bottom to the top. The karst at the bottom of the cycle was weak, with locally developed chip moldic holes. The upward karst reconstruction scope expanded, the dominant channel and the dissolution mottling began to emerge, and the karst process developed moderately; the upper karst system of the cycle cleaved and dissociated the bedrock, developed karst breccia, and exhibiting overdeveloped karst processes. (3) Under the control of exposure time, both high- and low–frequency cycles are developed in the study area, and the exposed surfaces of high–frequency cycles are mostly found in limestone, which is an “episodic” cycle interface, and the inner karst intensity is manifested as karst non-development→selective degradation of bioclastic debris→dominant channels and dissolution spots. In contrast, low frequency cycle interfaces are observed only at the top of slope sections or Maergou Section of the limestone, where inner karst intensity is manifested as dominant channels→dissolution spots→karst breccia. (4) The high-quality limestone reservoirs are mainly developed in the middle and upper parts of the quaternary cycle, corresponding to regions of moderate karst reconstruction area. In comparison, reservoir quality of the lower part of the cycle and the top part of the cycle became significantly worse. It is believed that the multi-stage karst in the early diagenetic stage not only controls the development and distribution of limestone reservoirs in the study area, but also greatly improves the reservoir and seepage capacity, which is the key factor for the formation of limestone reservoirs in Taiyuan Formation.