--> Overpressure And Seal Integrity Evaluation On Oligo-Miocene Carbonates Reservoir In The Madura Strait Area Of East Java Basin-Indonesia

AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

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Overpressure And Seal Integrity Evaluation On Oligo-Miocene Carbonates Reservoir In The Madura Strait Area Of East Java Basin-Indonesia

Abstract

Overpressure has been encountered in many wells in the Madura Strait area of the East Java Basin that were targeting the Oligocene-Miocene carbonates reservoir. In exploration context, knowledge of overpressure becomes critical as a potential control on seal integrity for Oligocene-Miocene carbonates reservoir. As the retention capacity (fracture pressure minus pore pressure) is getting smaller, the risk of having hydraulic failure of the top-seal increases, allowing hydrocarbons to escape. To evaluate the top seal integrity over the Oligocene-Miocene carbonates reservoir structures, it is critical to understand the retention capacity distribution and its relation with the overpressure distribution. Pore pressure measurements from Wireline Formation Tests, Drill Stem Tests, and kicks were used to map the distribution of overpressure. The magnitude of the overpressure tend to be higher at the central part of the Madura Strait area and coincide with thickest deposition of Wonocolo Formation sediments. In contrary, the magnitude of overpressure tend to be smaller at the western part of the Madura Strait area and coincide with thin deposition of Wonocolo Formation sediments. The velocity-density cross plot were used to identify the overpressure generating mechanism, whether it is due to disequilibrium compaction or unloading.

The retention capacity of the Oligocene-Miocene carbonates reservoir was calculated using pore pressure and leak-off test data. It is inferred that at a retention capacity of less than 1000 psi, the likelihood of hydraulic leakage is increased, which means more prone to seal failure. This is the case in the central part of the Madura Strait area where both carbonate build up type with high structural relief and deep water carbonate type with low structural relief were overlaid with greater thickness of Wonocolo Formation sediments. In contrary, where retention capacity is greater than 1000 psi, the likelihood of having hydraulic leakage is low. This is the case in the western part of the Madura Strait area where carbonate build up type with high structural relief were overlaid with thinner deposition of Wonocolo Formation sediments. As a conclusion, understanding the overpressure distribution, carbonate depositional environment, structural relief, and retention capacity in the study area can lead to a better seal integrity risking for future Oligocene-Miocene carbonates prospects.