--> Understanding Limestone Depositional Environment And Its Characteristic To Successfully Predict Pore Pressure In The Limestone: The Case Study From Offshore East Java Basin

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

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Understanding Limestone Depositional Environment And Its Characteristic To Successfully Predict Pore Pressure In The Limestone: The Case Study From Offshore East Java Basin

Abstract

Predicting pore pressure in limestone is still a big challenge in oil and gas drilling. The conventional prediction method relying on the relationship between porosity and effective stress is not applicable for limestone because the porosity in limestone is largely not the function of the effective stress. In this area, we found that, based on regional study, pore pressure in limestone is closely related to its depositional environment and characteristics. Consequently, understanding the ‘limestone geology’ could be used to predict overpressure prior to drilling in this area. The limestone reservoirs in the study area could be divided into two sequences: the shallower sequence, which is Plio-Pleistocene in age, and the deeper sequence, which is Miocene in age. Result of the analysis shows that pore pressure regime in the study area changes from hydrostatic to overpressure at the depth interval of ~500 – 1700 m. All limestone reservoirs (both Plio-Pleistocene and Miocene limestones) are located below these tops of overpressure. Interestingly, the pressure in the limestones could be as high as in the surrounding shale, less than its surround shale (but still overpressure), or even drop into hydrostatic condition. For the Plio-Pleistocene limestone, there are two pressure regimes in the limestone. The first one is the pressure in the limestone is as high as the surrounding shale, and the second one is the pressure in the limestone is hydrostatic. There is distinct boundary between them, and it is more or less directed west – east. The limestone located to the north from the boundary is hydrostatically pressured, while it is overpressure for the limestone located to the south of the boundary. The paleogeographic mapping of the Plio-Pleistocene sequences shows that the limestone in the northern area is a platform limestone which was deposited over wide area. Meanwhile, the limestone in the southern area was deposited in more isolated area. Therefore, pressure from surrounding shale could dissipate effectively in the limestone in the northern area, while it could not happen in the southern area because the limestone is confined in shale sequence. Moreover, the limestone in the northern area has a good connectivity with outcrop limestone in the Island of Madura, providing good pathway for pressure drainage. For the Miocene Limestone (better known as ‘Kujung Limestone’), the pressure in the limestone could be as high as its surrounding shale, less than its surrounding shale but still in overpressure condition, or drop into hydrostatic condition. The boundary between these limestone pressure regimes is not as clear as for the Plio-Pleistonce limestone because of limited well number. However, we still can observe that the limestone having overpressure as high as in surrounding shale is distributed in the eastern part of the study area, while the hydrostatically pressured limestone is distributed in the western part. Again, paleogeographic analysis shows that the limestone deposited in the eastern area is isolated limestone, while it is widely distributed limestone for the limestone located in the western area. The analysis of Sr-Sr and carbon-isotope isotopic data from hydrostatically pressured limestone also shows that the limestone has experienced strong and widespread karstification in the past, thus also providing pathway for pressure drainage. As a summary, we have demonstrated that the pressure regime in the limestone has a strong relation with its depositional system and characteristics. Since predicting pore pressure in limestone by using wireline log and seismic is still an ongoing research, we still able to predict pore pressure in limestone by understanding the ‘limestone geology’. The case from offshore of the East Java Basin is a good example on how to elaborate geological information to predict pressure regime in limestone.