Abstract
The karst fracture-cave oil and gas reservoirs of the Yingshan Formation in the northern slope of the Tazhong Uplift are well developed and have achieved good exploration results. However, the karst fracture-cave near the top of the Yingshan Formation is basically filled with mud fillings, which seriously affect the reservoir property, and the source and filling environment of the mud fillings have been unclear. Through the petrological and geochemical analysis of the fracture-cave fillings system in the typical wells of the Yingshan Formation, it has been found that (1) the fracture-cave fillings are mainly composed of a mixture of the bedrock dissolution dissociation particles, clay minerals, and calcite cements of the Yingshan Formation, and the content of each component in the different wells or in the cave interval is quite different. (2) Rare earth element analysis shows that the rare earth distribution pattern of the fracture-cave fillings is similar to the bottom marlstone of the Lianglitage Formation, indicating that the fracture-cave fillings should be mainly derived from the early seawater of the deposition during the Lianglitage Formation. (3) Cathodoluminescence, trace element analysis, and previous studies have shown that the formation and fillings of the fractures and caves mainly occurred in the hypergene period, which had the characteristics of an oxidized environment, and that there are two filling effects. First, the limestone of the Yingshan Formation experienced the formation of karst caves due to meteoric freshwater dissolution during the exposure period, and the limestone of the Yingshan Formation was dissolved, resulting in some insoluble clay and residual limestone gravel particles brought into the cave by the meteoric freshwater for filling. Second, the seawater transgression also played an important role during the deposition of the Lianglitage Formation. The clay content in the seawater was high during the early deposition of the Lianglitage Formation, which led to the clay being brought into the caves by the seawater during the deposition of the Lianglitage Formation for further filling; at the same time, calcite deposited into the caves with the clay. The above research promotes the study of the formation mechanism of the karst cave reservoir in the Yingshan Formation and has important theoretical significance for the guiding of the next oil and gas exploration in this area.
Highlights
Carbonate karstification can form rich pores, caves, and fractures [1], in which the buried subsurface can become a high-quality reservoir space for oil and gas [2]
The limestone of the Yingshan Formation experienced the formation of karst caves due to meteoric freshwater dissolution during the exposure period, and the limestone of the Yingshan Formation was dissolved, resulting in some insoluble clay and residual limestone gravel particles brought into the cave by the meteoric freshwater for filling
Studies have shown that the formation of the karst reservoirs in the Yingshan Formation in this area is mainly related to the short-term exposure of the karst in the Caledonian period, and a large number of fracture-cave reservoirs were formed by karstification [19,20,21]
Summary
Carbonate karstification can form rich pores, caves, and fractures [1], in which the buried subsurface can become a high-quality reservoir space for oil and gas [2]. Some studies have found transient, exposed karst reservoirs in the exploration of the Yingshan Formation of the Lower-Middle Ordovician in the northern slope belt, based on the discovery of the typical, buried hill karst reservoirs in Tazhong Uplift [14,15,16,17,18]. Studies have shown that the formation of the karst reservoirs in the Yingshan Formation in this area is mainly related to the short-term exposure of the karst in the Caledonian period, and a large number of fracture-cave reservoirs were formed by karstification [19,20,21]. The source, the filling period, and the formation environment and its relationship with the karstification of these mud fillings have always been the difficulties in the reservoir research in this region
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