Tectono-sedimentary characteristics and controlling factors of the lower-middle Cambrian gypsum-salt rocks in the Tarim Basin, Northwest China

Abstract

Due to their excellent sealing capacity, gypsum-salt rocks are often associated with hydrocarbon accumulations. In the Tarim Basin (Northwest China), exploration activity has demonstrated that the Cambrian gypsum-salt rocks played a key role in the development of hydrocarbon plays, but there is still a lack of research on them. This study utilizes geological data coming from outcrops, well logs, rock cores, and seismic reflection profiles to quantify the distribution and tectono-sedimentary characteristics of the Cambrian gypsum-salt rocks in the Tarim Basin, and decipher the controlling factors governing their deposition. The gypsum-salt rocks are present in the Lower Cambrian Wusonger and the Middle Cambrian Awatage Formations, which are distributed around the Aman and Awati depressions, two structures inherited from the Ediacaran intracontinental rift, and the Bachu uplift. These Formations are mainly composed of salt, gypsum, gypsum halite, gypsum dolomite and gypsum mudstone, and are usually deposited in medium-thin beds. The total and single layer thicknesses of gypsum-salt rocks in the north of Bachu uplift are greater than that in the south. These lithofacies can be associated with different environments of deposition, with the Bachu uplift interpreted as supratidal sabkhas and evaporation lagoons and the Aman-Awati depressions as large-scale evaporation salt basins with a bull's eye architecture of halite-gypsum-dolomite. Carbon and oxygen isotopes indicate an overall fall in relative sea level from the late Early Cambrian to the Middle Cambrian in the Tarim Basin, accompanied by higher-frequency oscillations. Influenced by this continuous decline of sea level, the eastern platform margin transformed from gentle ramp to progradational rimmed margin and blocked the water exchange between the open sea and the platform. The southwest side of the platform was characterized by a stable gentle ramp margin, which was flooded by seawater during rise of sea level. In addition, the development of the intraplatform shoals further provided morphological barriers for the lagoons in the Bachu-Tazhong uplift belt. The Aman-Awati depressions continued to subside during the Early and Middle Cambrian and became the main loci of salt deposition, whereas the depressions and half grabens formed under the tensile tectonic regime in the Bachu-Tazhong uplift belt controlled the development of evaporation lagoons and gypsum-salt flats. This study constrained the distribution and evolution of gypsum-salt caprocks, and thus provide new insights for the sub-salt and inter-salt hydrocarbon exploration in the Tarim Basin.