Tracking Hydrocarbon Reservoirs Within Cambrian Dolomitic Wedges in Northern Tarim Basin, NW China: Implications for Petroleum Exploration in Ultra-Deep Strata

Deeply buried carbonate strata have been recognized as significant exploration targets in recent decades. Great breakthroughs have been found in dolostones of the Lower Cambrian Xiaoerbulake Formation in the northern Tarim Basin, which shed light on global petroleum prospecting in deep Cambrian strata. However, sedimentary architectures of favorable hydrocarbon reservoirs within the Xiaoerbulake Formation remain unclear, hindering further exploration work in this region. In this study, a combination of petrographic, sequence stratigraphic, and C-O-Sr isotopic analysis was conducted for Cambrian dolomites intersected by ultra-deep drill-holes TS5, LT1, and YS2, in order to characterize the depositional architecture of the Xiaoerbulake Formation and infer favorable targets for petroleum exploration. Four high-frequency sequence stratigraphic units are identified within the Xiaoerbulake Formation (SSQ1–SSQ4). The three deep drill-holes occupied different positions within the same progradational depositional system. LT1 was located closest to the rollover and high-energy shoal belt, which contains favorable petroleum reservoirs. The C-O-Sr isotopic ratios, rare earth element distribution patterns and moderate dolomite ordering show that dolomitic reservoirs within the Xiaoerbulake Formation were formed during the early marine/shallow-burial diagenesis and may have been modified by basinal fluids. The outcomes of this study imply that ultra-deep Cambrian strata have the potential to preserve complete prograding carbonate wedge architectures that are favorable for petroleum exploration in the Tarim Basin, and possibly elsewhere in the world.