Seismic-informed carbonate shelf-to-basin transition in deeply buried Cambrian strata, Tarim Basin, China

Abstract

We develop an enhanced interpretation workflow to improve facies interpretation in ultradeep (5000–8000 m), high-velocity (6000–7000 m/s) carbonate rocks using low-frequency (as low as 18 Hz) seismic data. Our study area includes two seismic surveys north and south of the Cambrian platform margin that extends more than 500 km in strike direction. Relative impedance (−90° data) and frequency fusion are applied to predict impedance and lithology in carbonate platform-basin architecture. Outcrop seismic modeling clarifies the facies architecture of subtle subseismic clinoforms. Stratal slices are made for seismic geomorphology. We recognized eight seismic sequences and seven seismic facies in 1200-m prograding sediments. As a strongly prograding accretionary margin, the stratal relationships and transitions from platform interior to shelf margin to slope and basin are consistent throughout the basin. Platform interior is characterized by intertidal, supratidal, and salt flat facies; the shelf margin develops microbial reef and shoal facies; the slope and basin facies are dominantly gravity flow and deepwater sediments. The northern and southern areas of the margin, however, are characterized by different slope-to-basin geomorphology and environments. In the north, the shelf edge was cemented, with little reworking of shallow-water sediments to the slope and basin. Gravity failure–related facies, such as gullies and slump aprons, dominate in the slope. No turbidite fans are found in the sediment-starved deep basin. In the south, a series of submarine channels transported shallow sediments long distance to form slope fans and basin-floor fans. Turbidite sediments contain a significant amount of low-impedance sediments from the shelf edge. The syndepositional tectonic trench faults, by initiating channels, are hypothesized to be a new mechanism for sediment transport in a deepwater environment. Such a mechanism provides a clue to help researchers recognize and map deepwater reservoir facies in the basin.