Hydrocarbon Accumulation Differences in the Deep Ordovician Reservoirs of the Tahe Oilfield and Its Peripheral Regions, Tarim Basin, Western China

Abstract: The Tahe Oilfield in the northern Tarim Basin contains one of China's largest Ordovician carbonate reservoirs. Influenced by multi-phase tectonic uplift and subsidence, significant differences exist in hydrocarbon accumulation between the Tahe area and its periphery. This study focuses on the Ordovician Yingshan (O₁₋₂y) and Yijianfang (O₂yj) formations in the Tahe and surrounding areas. Integrating systematic analysis of source rocks, structure, reservoirs and fluid inclusion, the diagenetic processes and evolutionary sequences of the target strata were clarified, the phases and timing of hydrocarbon charging were determined, and a hydrocarbon accumulation model was established to identify the controlling factors behind differential accumulation. The key findings are as follows:

(1) Hydrocarbon properties and reservoir types vary across the study area. Crude oil density increases—and oil becomes heavier—from south to north, whereas the dryness coefficient and maturity of natural gas gradually increase from northwest to southeast. Reservoir types transition concentrically from south to north in the following sequence: dry/wet gas reservoirs → condensate gas reservoirs → volatile oil reservoirs → light oil reservoirs → medium oil reservoirs → heavy oil reservoirs.

(2) Fluid inclusions exhibit distinct characteristics in different wells. In the Yingshan Formation, blue inclusions dominate in Well YQ8 (East Yuqi) and Well TS3 (Deep Tahe), indicating high-maturity hydrocarbons. Well YQX1 (West Yuqi) contains a mixture of blue, yellow, and orange inclusions, reflecting the coexistence of high- and low-maturity hydrocarbons. Well TP18 (Tuoputai) is dominated by yellow~green inclusions, corresponding to medium~low maturity hydrocarbons. In the Yijianfang Formation, Well YJ1X (Yuejin) contains both blue and yellow~green inclusions, representing high and medium~low maturity levels.

(3) Multiphase hydrocarbon accumulation is evident, with significant variation in charging timing among wells. In the Yingshan Formation, Well YQ8 experienced three accumulation phases: Middle Hercynian (338~305 Ma), Middle Yanshanian (130~111 Ma), and Late Himalayan (22~16 Ma). Well YQX1 underwent a single phase during the Middle~Late Himalayan (31~6 Ma). Well TS3 recorded three phases: Late Caledonian (458~454 Ma), Early Yanshanian (191~173 Ma), and Late Himalayan (22~18 Ma). In the Yijianfang Formation, Well TP18 had three accumulation phases: Indosinian (233~210 Ma), Middle Yanshanian (138~123 Ma), and Late Himalayan (21~13 Ma); Well YJ1X experienced two phases: Middle Yanshanian (134~117 Ma) and Late Himalayan (22~12 Ma).

(4) Differential hydrocarbon accumulation is jointly controlled by multiple factors. Variations in hydrocarbon generation and evolution of source rocks determine the fundamental reservoir types and fluid properties. Differences in tectonic evolution directly influence the phases and timing of hydrocarbon accumulation, with multiphase tectonic activity serving as the primary driver of the complex hydrocarbon distribution patterns observed in the study area.

These results provide a theoretical foundation and technical reference for further exploration of hydrocarbon accumulation in deep to ultra-deep strata.

Keywords: Fluid inclusions; Differential hydrocarbon accumulation; Ordovician; Tahe area; Tarim Basin