Observations on Hydrocarbon Migration, Jasmine-Ban Yen Field; Block B5/27, Gulf of Thailand

Abstract

Abstract Detailed knowledge of fill-spill history and charge entry points to fields is rarely available, due to lack of suitable data sets and methodologies. This paper describes the application of a reservoir geochemical work flow (multi-variate statistical analysis of geochemical data) to unravel the fill history of a highly complex oil field in the northern Gulf of Thailand, and the implications of these results in assessing charge risk in adjacent and near-field prospects. The Jasmine-Ban Yen field, Pattani Trough, Gulf of Thailand, produces from stacked Middle to Late Miocene clastic reservoirs, draped over a highly faulted structural nose. In an earlier study, 59 oils from across the field underwent standardised fingerprinting, biomarker and bulk isotope analysis. Here, geochemical parameters considered resistant to secondary processes such as biodegradation, underwent hierarchical cluster analysis and classification into fluid families. Distinct families potentially represent fluids that share a common history. The results were synthesised with spatial information, seismic data, reservoir pressures, petroleum systems modelling, and observations drawn from the field's production history, to elucidate the fill-spill history of the field. All oils were expelled from similar lacustrine organofacies at similar maturity, which is broadly consistent with a single source pod charging the field. The closest mature kitchen is thought to be located in the Northern Pattani Trough, some 20 to 25 km to the south. A sub-regional Middle Miocene lacustrine seal, the "hot shale," focusses oil into the Jasmine-Ban Yen field, and forms the seal for 30% of the STOIIP. Fluids also occur in reservoirs above this seal, which could be emplaced either through vertical fill and spill via high offset faults, possibly aided by locally high CO2 increasing buoyancy pressure by formation of a gas cap, or laterally, via spill from adjacent fault blocks. Detailed knowledge of charge history remains elusive; however, the occurrence of consistently different fluid families above and below the hot shale seal, with fluids below represented by consistent families over a lateral distance of 12 km, supports an interpretation of multiple entry points into the field. Aromatic maturity parameters indicate that four Ban Yen samples are of slightly elevated maturity, suggesting that late charge accesses the field above the hot shale. The possibility that the differences between families are related to biodegradation was investigated and discarded. Families probably represent discrete, lateral spill pathways reflecting multiple charge entry points and are differentiated by subtle variations in organofacies related to oxicity and contribution from plant material. Comparable migration above and below the hot shale into B5/27 is a possibility, and exploration prospectivity is risked accordingly. Placing statistically derived fluid families into a spatial, geological and production context enables unravelling of migration vectors in complex fields. Furthermore, inferences may be drawn from such a study that can help guide risk assignment to offset exploration prospectivity.