Reactive Transport Modeling of Reflux Dolomitization of Carbonate Platforms: Enlightenment from Yingshan Formation in Shunnan Area, Tarim Basin

Dolomite plays an important role in carbonate reservoirs. The topography in the study area creates conditions for reflux dolomitization. The northeastward paleogeomorphy during the deposition of the Yingshan Formation was favorable for reflux dolomitization. Furthermore, the petrological and geochemical evidence indicated that the formation of finely crystalline dolomites was penecontemporaneous to sedimentation. The content of powder crystal dolomites increases from grainstone, to packstone, to mudstone. Previous studies only analyzed the origin of dolomites based on traditional geological methods, but did not analyze the spatial influence of reflux dolomitization on the reservoir quality. In this study, the reflux dolomitization of platform carbonate sediments was evaluated using three-dimensional reactive transport models. The sensitivity of dolomitization to a range of intrinsic and extrinsic controls was also explored. The reflux dolomitization involves replacement dolomitization and over-dolomitization. The porosity change is the result of the abundance change of dolomite and anhydrite. The fluid flow pattern in the model is related to the injection rate and geothermal gradient. According to the spatial and temporal change of mineral, ionic concentration, and physical property, the reflux dolomitization could be divided into five stages. From the sensitivity analysis, high permeability promotes dolomitization only in the initial stage, while low permeability and high porosity means stronger dolomitization. Besides, the injection rate, reactive surface area (RSA), geothermal gradient, and brine salinity are all proportional to the dolomitization. Differently from porosity change, the permeability change is concentrated in the upper part of the numerical model. The location of “sweet spot” varies with the locations of change centers of porosity and permeability. In the stage-1 and 4 of dolomitzation, it overlaps with porosity and permeability growth centers. While in the stage-2, 3 and 5, it lies between the porosity and permeability growth/reduction centers.

The Source of Fracture-Cave Mud Fillings of the Ordovician Yingshan Formation and Its Paleokarst Environment in the Northern Slope of the Tazhong Uplift, Tarim Basin, China: Based on Petrology and Geochemical Analysis

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.

Reservoir forming conditions and exploration potential of Lower Paleozoic carbonate rocks in Gucheng area, Tarim Basin

The Gucheng region is oil and gas accumulation area of Tarim Basin，dominated by beach controlled lithologic oil and gas reservoirs，which has superior oil and gas accumulation conditions and broad exploration prospects. The lower Paleozoic strata in the Gucheng region are the Carboniferous, Ordovician, and Cambrian systems from top to bottom. The Silurian, Devonian, and Permian systems are missing. The lower Ordovician – Cambrian are the main exploration target layers in the area. There are four reservoirs: the Ordovician group, the Yingshan group, the Penglaiba group and the Cambrian group in The Gucheng region, The upper part of the Yijianfang formation and Yingshan formation are limestone reservoir, which is a set of potential reservoir. The lower part of the Yingshan formation, the Penglaiba formation, and the Cambrian formation developed three sets of dolomite reservoirs. The distribution of oil and gas in Gucheng region was mainly found in Yijianfang formation, Yingshan formation, Penglaiba formation and Cambrian of the Ordovician. The Yingshan group and the Hanwu group were the main exploration target layers. Yingshan formation and Cambrian are the main exploration targets, and Yijianfang formation and Penglaiba formation of Ordovician can be used as the concurrent exploration targets. Gucheng region can form two major realistic exploration areas: dolomite reservoir of Ordovician and reef beach of Cambrian. The Ordovician dolomite reservoirs are mainly searched for the inner shoal of the platform edge high-energy facies belt, and the Cambrian system is mainly search for the high-energy reef beach far away from the platform edge facies belt in the north of cherchen fault.

Predicting and 3D modeling of karst zones using seismic facies analysis in Ordovician carbonates of the Tahe oilfield, China

Karsted carbonates of the Ordovician Yingshan Formation represent significant hydrocarbon reservoirs in the Tarim Basin, China. Due to the geologic complexity of the formation, realistically predicting and modeling karst zones and rock properties is challenging. This drives the need to apply diverse techniques for building a suitable geologic model. We have developed a static model approach that uses fully automated seismic facies classification processes for predicting and modeling patterns associated with karst elements. Our method uses a seismic attribute and well logs as input data. We initially processed a seismic facies volume using the hierarchical clustering technique. This is based on seismic attribute values that take into account an optimal number of classes. The outcome reveals various patterns illustrated with low amplitudes highlighting the geomorphology of paleokarst elements. Simultaneously, a seismic traces map of the karsted interval was processed using the hybrid clustering technique conducted on seismic trace shape. In this case, the karst facies was extracted from the output and used as secondary input data in trend analysis of the model. Both outputs obtained from clustering techniques are processed in a volume of the most probable facies, which delineate the karst patterns. The results of the modeling process are visualized in various time slices and cross sections, appropriately recognizing the relationship of estimated patterns with karst zones. We have evaluated the karstification thickness and porosity map obtained from the 3D model that detail a reasonable connectivity between karst elements. This is based on the paleogeographic location and type of filling, as well as the dissolution development along the main striking faults. Finally, our method outputs a logical model of karst zones located within the host rock, which reduces the uncertainty and identify nonperforated segments.