--> Quality Reservoir Bed Development and Karst Cycle of Carbonate Rocks Buried Hill: A Case Study From Bozhong Sag

AAPG ACE 2018

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Quality Reservoir Bed Development and Karst Cycle of Carbonate Rocks Buried Hill: A Case Study From Bozhong Sag

Abstract

The carbonate rock reservoir of Paleozoic buried hill is the main exploration layer in Bozhong Sag, the center of Bohai Bay Basin. The sectional feature of gas reservoir in vertical profile is revealed by drilling wells, suggesting that karst reservoir development is zonation. It is the key to predict quality reservoir distribution that understanding cycle feature of ancient karst in the area.

According to core, thin section, electrical, spectrum logging U value, C/O isotope, trace element Sr, etc., combined with the hydrodynamic magnitude and direction, karst reservoir can be divided into four zones in vertical profile. The difference of lithologic-electric property and denudation intensity were closely related to development of quality reservoir.

The surface weathering zone and horizontal underflow zone: higher natural gamma for 50-80API, showing zigzag, relatively low resistivity about 200-500Ω●m, too large U relative value with 0.5-0.8, small 13C (-14‰~2‰) and 18O (-12‰~-10‰), larger 87Sr/86Sr with an average of 0.7115, feature with mixed freshwater, reservoir space is pore-crack type. The surface weathering zone is micritic limestone, porosity about 5%-7%; horizontal underflow zone develops grainstone, prone to strong dissolution, porosity of 6%-10%, with a maximum of 15%, good physical properties.

The vertical percolating zone and deep slow flowing zone: low natural gamma values with 20API, flat curve, abnormally high resistivity beteween 1000 and 2500Ω●m, even up to 4000Ω●m, smaller U relative value with 0.2, large 13C (-0.8‰~-0.4‰) and 18O (-8‰~-5‰), low Sr ratio with 0.7093, being the characteristic of seawater, the reservoir space is fracture type. The vertical percolating zone is compacted limestone, goes against corrosion, porosity less than 1%, poor physical properties; deep slow flowing zone generates dolomitization, porosity around 3%, better property than vertical percolating zone.

It shows that reservoir development is controlled by karst zoning. The quality reservoir is mainly distributed in the surface weathering zone (20m under unconformity) and horizontal underflow zone (50 to 200m beneath unconformity). The potentially quality reservoir is developed in dolomite-existing area of deep slow flowing zone. In the guidance of the karst zoning models, a breakthrough of gas exploration has been made with over 50 billion cubic meters of gas reserves in Paleozoic deeply buried hill of Bozhong 21-22 tectonic province in southwestern Bozhong sag.