Improved Massive Dolomite Mapping by Integrated Forward Diagenetic and Geological Modeling

Abstract

Stratigraphically discordant massive dolomite bodies of the Upper Jurassic have long been documented because they strongly affect reservoir quality. Dolomitization is affected by varies factors such as original depositional texture, dolomitizing fluid, dolomitization timing and types, and previous diagenetic stages, which can make dolomite bodies either flow conduits or barriers. Therefore, understanding the complex diagenetic system and the distribution of the massive dolomite are extremely important. In this study, we integrated forward diagenetic and geological modeling following 4-step approach: (1) detailed 3D geologic modeling to delineate the spatial distribution of the massive dolomite; (2) calculation of the effects of dolomitization on reservoir quality; (3) property modeling to predict the spatial distribution of reservoir quality; (4) integrate geological and diagenetic forward modeling to improve the understanding of the dolomitization system and its impact on reservoir quality. Modeling results indicate that: (1) In general, dolomitization can be divided into two phases, replacement and pore-filling. During the replacement phase, porosity preservation is the dominant process, while during the pore-filling phase porosity decreases sharply with the increase of dolomite volume fraction. Overall, the replacement phase improves reservoir quality, while the pore-filling destroys it; (2) The massive dolomite is heterogeneously distributed with an overall regional trend of decreasing dolomite content southwards; (3) two episodes of dolomitization are likely to occur, supported by multiple types of data, the first is driven by the tectonic compression and developed adjacent to salt basins, whereas the second is related to late hydrothermal dolomitization overprinting the early dolomite. This integrated forward diagenetic and geological modeling approach helps to better understand the dolomitization mechanisms and regional diagenetic system, by improving the mapping of the massive dolomite and the prediction of reservoir quality.