Tectono-sedimentary evolution and hydrocarbon reservoirs in the Early Cretaceous Tanan Depression, Tamtsag Basin, Mongolia
The responses of depositional patterns to extensional faulting during the rift cycle are discussed in this article, based on an integrated analysis of 3D seismic reflections, wire logs and cores from the Early Cretaceous Tanan Depression in the Tamtsag Basin, Mongolia. The synrift succession comprises four third-order sequences (SQ1-4). A four-stage fault evolution is defined, including early synrift SQ1 (initial isolate faults), early synrift SQ2 (rapid fault propagation), climax synrift SQ3 (maximum fault relief) and late synrift SQ4 (minor fault activity) stages. The initial isolated fault stage is defined by separated and small-sized half-grabens associated with short-dispersal alluvium. In contrast, the rapid fault propagation stage is characterised by much longer dispersal systems of fan-delta deposits. As rifting progressed, a considerable increase in the basin subsidence occurred and the maximum fault displacement led to a dustpan basin geometry, the coalescence of depocentres and extensive deep-water deposits. The observed both sediment starvation and balanced-filled patterns in SQ3 indicate that caution must be exercised when attempting to correlate reservoirs and source rocks since variations in basin-filled patterns would be enhanced by the displacement localisation on several major faults during the significant basin formation. During the early period of the climax synrift stage, footwall-sourced systems were prominent, but drainages in prior relay zone sites were mostly decayed or submerged. In response to the subsequent degradation of the footwall sources, axial-sourced braided deltaic systems became more important during the later period. An abrupt decrease of faulting at the end of the rift cycle gave a rise to an increased sediment supply and long-dispersal axially-sourced systems in the Tanan Depression. Petroleum exploration shows that fan-delta front sandstones in SQ2 are the most prospective hydrocarbon reservoirs, whereas the nearshore and offshore sub-lacustrine fan deposits in SQ3 have proved to be secondary potential oil-bearing reservoirs. This study indicates that sediment dispersal patterns would be significantly re-organised by the differential faulting displacement patterns and varied sediment potentials during the different synrift stages. The rapid fault propagation allowed the fault arrays to reach their nearly maximum length during the early period of the basin evolution, while the displacement remained almost constant. This fault growth pattern tends to establish large-scale topographic lows including overlapped transfer zones and hanging dip-slope. Hence, major drainage catchments and long-dispersal depositional systems of sand-rich deposits are facilitated during this early fault propagation stage, which may be a key aspect for the tectono-sedimentary evolution in rift basins. In addition, the fault and sedimentary relationships defined here may be used as a model when studying less data-rich rift basins elsewhere.