The Detailed Palaeogeomorphologic Restoration of the Ordovician Weathering Crust in the Northern Jingbian Gas Field: Methods and Applications into Reservoir Properties and Gas Accumulation

Abstract

Palaeogeomorphology plays a crucial role in the development of sedimentary facies and the reservoir distribution in the late period of basin evolution, and to a certain extent, controls the assemblage of the late hydrocarbon reservoirs and caprock formations. Therefore, it has a function in controlling the enrichment of oil and gas. Caledonian orogeny uplifted the Ordovician Majiagou Formation in the Ordos basin and thus led to the formation of the weathering crust reservoirs in the process of weathering, erosion and leaching for over 130 Ma. Such palaeogeomorphology represents the background of the palaeokarst, which has an essential influence on the formation of reservoir rocks. Based on Precarboniferous paleogeologic map plotted by 305 wells, and the thickness of bauxite in 272 wells, methods of residual thickness and moulage are applied to reconstruct the palaeogeomorphology in the studied area. In combination with the thickness of horizon to the weathering crust, the criteria for the second order palaeogeomorphologic units were proposed. They are karst highland, karst slope and karst basin, which can be further subdivided into the third order palaeogeomorphologic units of remnant hill, depression, gentle hill, shallow depression and erosional grooves. According to the criteria and refined identify results of ancient grooves using geological data of 272 wells, palaeogeomorphology of the Ordovician weathering crust in the northern Jingbian gas field has been restored accurately. The results show that karst highland developed in the northwest part of the studied area, with remnant hill and depression occurring locally. Karst slope was present in the central and east, predominatly in the form of gentle hills and shallow depressions, with 5 nearly EW-trending ancient erosional grooves developed locally. Finally, integrated data of reservoir property with gas production test implied that the remnant hill in the karst highland, the gentle hill in the karst slope and the two sides of the erosional grooves were favorable positions for gas accumulation.

Reconstructing large‐scale karst paleogeomorphology at the top of the Ordovician in the Ordos Basin, China: Control on natural gas accumulation and paleogeographic implications

“…Therefore, this method should be applicable to the upper Majiagou Formation, deposition of which was spatially uniform. Specifically, for the residual thickness method, a layer underlying the unconformity surface should be firstly selected as the isochronous reference surface, and then, the residual thickness between the reference surface and the unconformity surface should indirectly reflect paleogeomorphology. The moldic thickness method requires stable synsedimentary tectonics during the filling process over the denuded strata, which is applicable to the Carboniferous Benxi Formation .…”

“…Specifically, for the residual thickness method, a layer underlying the unconformity surface should be firstly selected as the isochronous reference surface, and then, the residual thickness between the reference surface and the unconformity surface should indirectly reflect paleogeomorphology. The moldic thickness method requires stable synsedimentary tectonics during the filling process over the denuded strata, which is applicable to the Carboniferous Benxi Formation . In this method, a layer overlying the unconformity surface should be firstly selected as the isochronous reference surface, and then, the thickness of the overlying strata should represent the paleogeomorphology given the mirror‐image relationship between the overlying strata and the unconformity surface.…”

“…In this method, a layer overlying the unconformity surface should be firstly selected as the isochronous reference surface, and then, the thickness of the overlying strata should represent the paleogeomorphology given the mirror‐image relationship between the overlying strata and the unconformity surface. Therefore, residual thickness (H) and moldic thickness (h) data as acquired from different exploration wells can be used to determine paleogeomorphology . Large values of H and h generally indicate good preservation of underlying strata and large filling thickness of overlying strata on top of the unconformity, suggesting a geomorphic low.…”

Petrography and facies distribution of Middle Ordovician Ma 51 + 2 tight dolomite reservoirs in the Ordos Basin, Central China

“…These units are expected to provide oil and gas resources in addition to the producing sandstone reservoirs (Sun et al., 2016; Wei et al., 2017a, 2017b). However, in the eastern Ordos Basin of tight carbonate gas exploration, the petrology and facies distribution of the Ma 5 1 + 2 reservoir are obscured by karstification (Hong et al., 2018; Wei et al., 2015; Xiong et al., 2016). Furthermore, the depositional environment is debated, and carbonate shelf (Hou et al., 2003), barrier lagoon (Zhou et al., 2011), tidal flat (Chen et al., 2018; Xie et al., 2013), and restricted evaporite platform (Shi et al., 2009; Zhang et al., 2015) environments have all been suggested.…”

Discovery of a shoal-controlled karst dolomite reservoir in the Middle Permian Qixia Formation, northwestern Sichuan Basin, Southwest China

“…Carbonates play important roles in global petroleum exploration, and karstification and dolomization, as common diagenesis in such rocks, have significant implications for the formation of high-quality reservoirs through diagenetic alteration (Braithwaite et al., 2004; James and Choquette, 1988; Moore and Wade, 2013). As a consequence, karstification and dolomization have attracted continued attention from petroleum geologists for decades (Cao et al., 2015; Conliffe et al., 2012; Davies and Smith, 2006; Feng et al., 2013; Lü et al., 2011; Wei et al., 2015). The Middle Permian Qixia Formation in the Sichuan Basin of southwestern China is one of representative cases (Tian et al., 2014; Wang et al., 2013).…”