Depositional model, facies analysis and reservoir quality in a wave-modified clastic lacustrine system: The early cretaceous Tongbomiao formation of the Tanan Depression, Tamtsag Basin, Mongolia

Abstract

The Tanan Depression has experienced multi-phase tectonic movements and formed various types of reservoirs, which has brought some difficulties to proper oil exploration. This study aims to characterize the depositional facies and petrophysical properties of clastic reservoirs in the Tanan Depression and to determine the most prolific areas for exploration based on the analyses of core samples, thin sections, logging data, petrophysical properties, seismic profiles, and paleontological data. A wind-wave controlling sedimentary model and seven primary depositional facies, namely: 1) alluvial fan; 2) braided delta plain; 3) braided delta front; 4) wave-modified fan-delta front; 5) shoreface; 6) storm deposits; and 7) sublacustrine fan, were revealed in this study. The predominant wind direction during the deposition of the upper Tongbomiao Formation was from the southeast (SE), and the resulting wave-modified sandbodies mainly extended along the paleoshore line of the lake in a NE-SW direction. Facies of weakly-to not-modified depositional settings (alluvial fan, braided channel, braided delta front, and sublacustrine fan) were mainly characterized by high matrix content, poor grain sorting, and poor maturity. Conversely, facies related to wave-dominated depositional settings (wave-modified fan delta front, shoreface and storm deposits) are characterized by low matrix content, higher grain sorting, and higher maturity. A positive correlation between reservoir quality and wave-modified intensity of sandstones is evidenced in the study area. Sandbodies that have experienced wave modification, early carbonate cementation, and dissolution served as the main reservoirs for oil accumulation. Therefore, future oil exploration in the Tanan Depression should focus on clastic sedimentary bodies reworked by waves. Moreover, the facies distribution model and high-quality reservoirs forming mechanism in the Tanan Depression could provide a significant reference for oil exploration and development in other similar basins.