Sequence Filling Patterns and Petroleum Implications of Lower Cretaceous Series in South Tamtsag Depression

Abstract

Sequence stratigraphic study of Lower Cretaceous strata in the South Tamtsag Depression of Mongolia identifies one second-order and four third-order sequences. These sequences are controlled by syndepositional fault activities at the slope-break zone of the lacustrine basin. At the initial rift stage of the basin, creation of accommodation space outpaced sediment accumulation and the lake level fell below the structural slope-break zone. Sequences formed in this stage can be divided into lowstand, transgressive and highstand system tracts. At the main rift stage, lake level did not fall below the structural slope-break zone and sequences formed in this stage consist of transgressive and regressive system tracts. Vertical stacking patterns within sequences are expressed by cyclic occurrences of alluvial fan, fan delta and delta deposits in the gentle slope belt located in the western part of the basin, and fan delta, nearshore subaqueous fan and lake deposits in the steep slope belt in the eastern part of the basin. The distribution of sandstone units in the sequences, which are the major target reservoirs, can be easily recognized through the combination of sedimentology analysis and interpretation of seismic reflection profiles. The slope-break zone of the lacustrine basin, which can be identified from seismic reflection profiles, played critical roles on the thickness and lateral extension of the sandbodies in the basin, and thus is the main controlling factor of petroleum reservoirs in the basin. Because mudstone and oil shale developed during the transgressive system tracts are hydrocarbon source rocks and cap rocks, deltaic sandstone units and incision channel fills are the most important petroleum reservoirs in the Tamtsag basin. Finally, this paper divides four types of subtle traps according to the petroleum accumulation model for future exploration.