Seal Capacity Evaluation by Using Mercury Injection Capillary Pressure Method Integrated with Petrographic Data: A Case Study in Green Dragon Oilfield, Offshore Vietnam

Abstract

Summary The recent drilling successes in Green Dragon structure in Nam con Son basin, offshore Vietnam have confirmed that this structure is potentially commercial discoveries with recoverable reserve nearly 41.8 mmbbl of oil based on wireline logging and MDT (Modular Formation Dynamic Tester) data. The seismic interpretation and prospect-time structure map reveal that Green Dragon structure is a 4- way dip anticlinical structure are thought to be result of transpression devirved from strike-slip or/and normal faulting. This anticlinical feature of the Green Dragon structure provided trap for series of hydrocarbon bearing sandstone units. According to petrophysical interpretation, total 12 commercial hydrocarbon bearing zones was identified. The hydrocarbon bearing zones were located below interbedded shale beds which worked as top seals. The existence of hydrocarbon accumulations indicates the sealing capacity of these inter-bedded shale formations holding hydrocarbon columns. However, the hydrocarbon thicknesses of reservoir units are different. This might be derived whether from difference in sealing capacity of top sealing beds or difference in hydrocarbon charging to each level. Hence, the top sealing capacity needs to be evaluated critically. This study presents an integrated approach using Mercury Injection Capillary Pressure (MICP) and petrographic analysis to assess the seal quality of the inter-bedded shale formations which are considered the intra-formation top seals of hydrocarbon bearing zones in Green Dragon structure. Based on the hydrocarbon column height at leak point derived from capillary pressure data, four seal types were identified. Furthermore, the result of scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis were interpreted to clarify the influence of clay minerals on seal capacity.