Sedimentary Evolution of the Paleocene–Eocene Carbonate Platform in the Offshore Indus Basin: Implications for Hydrocarbon Exploration

The offshore Indus Basin, situated at the convergence of three major tectonic plates in the eastern sea area of Pakistan, has undergone complex tectonic-sedimentary evolution processes throughout geological history. During the Paleocene–Eocene period, the basin was equatorially positioned, hosting a large-scale carbonate platform whose sedimentary characteristics and evolution remain unclear due to limitations in well drilling, as well as the quality and quantity of seismic data. This knowledge gap hinders in-depth oil and gas exploration. This study employs a combination of existing and newly acquired two-dimensional seismic data, along with well-to-seismic calibration, to identify seismic facies of the Paleocene–Eocene carbonate platform in the offshore Indus Basin. The internal plane and vertical distribution characteristics in different periods were depicted, and the sedimentary evolution history and the main controlling factors were analyzed. Six types of seismic facies were identified, with the mound or lenticular shape-medium weak amplitude-medium low continuous-medium low frequency seismic reflections corresponds to reef facies exhibiting favorable reservoir petrophysical properties. The carbonate platform in the study area is categorized into four sedimentary evolution stages: initial development, prosperity, recession, and inundation. The study correlates these stages with changing platform characteristics such as vertical growth, increased thickness of reef shoal facies, platform edge recession, and final inundation by seawater. Considering regional tectonic evolution, sea-level fluctuations, paleoclimate evolution, and the temporal and spatial distribution characteristics of the carbonate platform, the study suggests that a pre-existing ancient highland terrain shaped by magma intrusion limited spatial distribution, shape, and scale of the carbonate platform during the Paleocene–Eocene period. Sea-level changes and paleoclimate evolution influenced the platform shape. In the hydrocarbon accumulation process of the reef reservoir, the distribution of effective source rocks, migration pathways, and the spatial arrangement of reef reservoirs emerged as key control factors. The study identifies an isolated platform in central area lacking sufficient oil and gas filling, confirmed by drilling. Conversely, the southern area features numerous reefs with various scales, connected by northeast-trending faults serving as effective channels for hydrocarbon migration. This study provides essential insights into regional sea-level changes, driving mechanisms, and supports hydrocarbon exploration in the offshore Indus Basin.