Karst Hydrocarbon Reservoir Characterization and Sweet Spot Prediction in the Western Tazhong Oilfield, China

Abstract

The Ordovician carbonates in the Tarim Basin’s central uplift zone are crucial for oil and gas exploration. A comprehensive analysis has been conducted utilizing 3D seismic imaging, logging, core samples, and production tests to explore its potential. The results indicate that dissolution within the Ordovician reservoir, attributed to an intricate fault system involving karstification and faulting, enhances hydrocarbon storage. Both structural and dissolution-induced fractures are vital for efficient hydrocarbon flow. NE-SW strike-slip faults significantly impact the distribution of carbonate reservoirs, particularly in early karst strata with higher porosity. Seismic sections reveal three distinct reflection patterns: full, single peak, and chaotic. The forward seismic model shows that oil/gas saturation influences seismic energy. High-yielding wells are located in formations with full-waveform patterns and high energy, indicating high porosity. A strong correlation exists between Class I seismic phases and high-porosity zones. Daily production in these zones ranges from 30 to 70 tons, validating the evaluation methods. The Yijianfang formation excels in oil storage due to its karst features and numerous unfilled tectonic and dissolution fractures, offering ample storage and permeability. High production is seen in strata with porosity >4.5%, but fractures alone do not ensure high production. Structural activity, occurring mainly during the meso-deep burial stage, allows acidic solutions to permeate structural fractures, leading to further dissolution. Karstification is confined to depths within 80 m of the Yijianfang formation top, indicating significant exploration potential.