--> Enhancing Oil and Gas Production in Carbonate Turbidite Fields by the Study of High-Resolution Biostratigraphy, Facies and Fracture Variability: An Example From the NW Palawan Basin of the Philippines

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Enhancing Oil and Gas Production in Carbonate Turbidite Fields by the Study of High-Resolution Biostratigraphy, Facies and Fracture Variability: An Example From the NW Palawan Basin of the Philippines

Abstract

High resolution biostratigraphy in conjunction with detailed sedimentology and basin analysis was used to characterise the thick carbonate turbidite reservoir of the West Linapacan A oil field, NW Palawan Basin in the Philippines. Although the field produced 8.5 million barrels of oil from a fractured, carbonate turbidite reservoir, early high water cut led to the premature shut down of oil production. The main reservoir unit, the Linapacan Limestone, was modelled as having “layer-cake” stratigraphy despite the internal heterogeneity typical of turbidite deposits. This study aimed to identify heterogeneity or changes in continuity. The work has also helped identify differences with the completely undeveloped West Linapacan B Field, which is within tieback proximity. A complete biozonation of the sediments was performed to tie the fields in with the basin's framework, as well as constrain the ages of the formations. These aims also permitted the identification of any potential controls on the deposition of the Linapacan Limestone to be identified, as carbonate turbidite deposits are not fully understood due to the paucity of analogues. The fields were analysed using foraminiferal biozonations, high-resolution biostratigraphy, petrographic analysis and well log correlation. This study found the Linapacan Limestone had increased vertical heterogeneity and decreased lateral continuity with an increase in stratigraphic resolution. Carbonate grainsize, fabric and texture in addition to gamma response trends, indicates intense variation on a fine scale, while on a coarse scale a “layer-cake” stratigraphy becomes apparent. As such, this study found that a “layer-cake” stratigraphic model proved accurate on a scale >10 metres, while on a scale <10 metres, the model was inaccurate. Considering the high resolution stratigraphic analysis, three main hypotheses emerged to explain the observed decrease in productivity at West Linapacan B Field, 6 kilometres to the east: 1) the time-rock sequence is not continuous from A to B; 2) there is a lateral facies variation present between the A and B structures, that may affect reservoir quality; and 3) there is a variation in fracture density between the A and B fields. Apart from the above techniques, 3D seismic incoherence was used to interpret fracture density. Based on this investigation, the lateral continuity of the time-rock sequence was resolved. Well log and seismic results suggest that the Linapacan Limstone is continuous from A to B as an entire package, but experienced a significant variation in lithofacies, with A being dominated by wackestones-packstones and B by mudstones-wackestones. This disparity in lithofacies has been attributed to the location of the wells within different depositional settings of the same calciclastic submarine fan system. Furthermore, A experienced a higher fracture density than B, attributed to an increase in faulting at the A structure, in addition to a more fracture-prone assemblage of facies. These fractures are also more likely to be open at A due to a decreased confining pressure and a maximum present day horizontal shortening direction that is parallel with the dominant fracture trend. Hence, as A produced from fractures, B's decline in productivity during testing was most likely a result of a decrease in the presence of faults proximal to the well path. This work has enhanced the understanding of carbonate turbidites and calciclastic submarine fans, allowing for the potential petroleum systems located within to be more finely delineated.