Abstract
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.
Highlights
The actual reflux dolomitization simulation process is the mixture of Equations (7) and (8)
The topography derived from sedimentary facies and petrological and geochemical evidence all suggest that the Shunnan area is favorable to the occurrence of reflux dolomitization
Reflux dolomitization was conducted on a three-dimensional model using TOUGHREACT
Summary
The oil–gas output of the carbonate rocks accounts for 60% of total reserves [1,2], and half of the output came from dolomite. The production potentials of the dolomite reservoir or dolomitized reservoir are huge. Khufu Formation of the North Dome of Qatar and Jurassic Arab-D Formation of the Ghawar field were partly dolomitized [3]. In addition to the Sichuan and Ordos Basin, high-quality dolomite reservoirs have been found in the Yakela Oilfield and Lunnan
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