Structural And Stratigraphic Evolution Of The East Java Sea, Indonesia

© IPA, 2006 - 21st Annual Convention Proceedings, 1992 IPA 92-11.24 PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Twentv First Annuall Convention, October 1992 STRUCTURAL AND STRATIGRAPHIC EVOLUTION OF THE EAST JAVA SEA, INDONESIA P.J.E:. Bransden * S.J. Matthews ** ABSTRACT INTRODUCTION The East Java Sea displays excellent examples of Miocene to Present-Day structural inversion of a Palaeogene extensional/transtensional basin system. The structural geometry of these inversions has been analysed by a series of regional cross-sections whijch demonstrate significant lateral variation in structural style. This variation is interpreted to be a function of the fault shape and linkage which evolved during the extensional phase of basin development. Prediction of fault geometry has been aided by analogue-modelling of inverted extensional basins. The East Java Sea Basin is part of the system of backarc basins developed around the margins of the Sunda Shield, from North Sumatra to Central Sulawesi. Onshore East Java is well explored and has been producing oil since the late 19th century. Exploration of the East Java Sea began in the early 1970’s and has proven significant gas reserves (including Pagerungan and BD), and a number of currently sub-economic oil and gas finds. BP’s involvement in the East Java Sea Basin commenced in 1982 with Hudbay in the Madura Strait PSC. Currently BP holds the three Lombok PSCs (Sakala Timur, Satanger and Sailus), and is also a partner in the Kangean PSC, operated by ARBNI. Driving mechanisms of structural inversion have belen assessed by modelling the motion of the Eurasian, Indian Ocean, Australian and Pacific plates during the Neogene. The evolution of inversion within the East Java Sea is interpreted to be a function of the propagating collision of the Australian Plate with the Sunda Arc, and the resultant variation in relative convergence rate and direction of the subducting Indian Ocean Plate and the S.E. Eurasian Plate. Stratigraphic evolution is strongly influenced by relative sea level fluctuations, principally eustatic, within overall control of tectonic processes, particulairly inversion. The sediment fill is mainly fine-grained with alternating carbonate-dominated and clasticsdominated cycles. The use of wireline logs, seismic data and biostra.tigraphic data allows a detailed sequence stratigraphy to be developed, with both regionally and locally recognisable highstand, lowstand and transgressive systems. A well-imaged progradational carbonate platform margin exhibits late-stage growth and subsequent inundation which are used to calibrate locally derived relative sea-level curves. * BP Exploration Operating Co. - Jakarta * * BP Exploration Operating Co. - London The nine discoveries in the main basin area occur in eight different play types, largely a function of the stratigraphic variability. Although some areas of the basin are complex structurally, it appears that many well failures are due to difficulty in predicting the lateral distribution of reservoir and source facies. With the benefit of a large seismic database BP has undertaken regional basin analysis in order to improve understanding of the basin evolution and in particular the relationship between tectonics and sedimentation. The principal objective has been to identify the full extent of individual depositional systems (’systems tracts’), their seismic boundaries, and the structural and eustatic controls on their deposition. The resulting sequence framework has been used both to understand lithofacies distribution in known fairways and also predict potential new fairways. Basin Distribution and Tectonic Setting The regional tectonic and structural frameworK has been established by Hamilton (1979)