Abstract

Recent studies of the Tertiary tectonics of the Bawean area suggest that strain is regionally inhomogeneous and the area has been partitioned into several domains separated by major strike-slip faults and extensional zones. Major structural elements occurring from west to east are the Muriah Trough, the Bawean Arch, the Tuban-Camar Trough, the JS-1 Ridge, the Central Deep Depression, the Masalembo Basin and the North-Madura Shelf. Tectonic rotation of these domains, as well as their internal deformation, has occurred as a result of regional sinistral shear. Two major tectonic phases have been identified, one controlling the Paleogene rift-phase and the other the Neogene reactivation of the Bawean area.

The sedimentary fill of the Paleogene basin is closely related to its tectonic history. Doming and fracturing of the entire region during the Eocene/Oligocene was followed by a period of regional subsidence and tectonic quiescence lasting into the Early Miocene. Northeast-southwest trending grabens formed during the Middle Eocene and were filled with alluvial clastics, lateritic clays and lacustrine shales. Organic rich shales in this sequence provide the source rock for hydrocarbons throughout the region. Basement onlap began in the Late Eocene to Early Oligocene with the deposition of transgressive basal marine sandstones and limestones including reefs.

Facies distribution in the Neogene is controlled also by the positions of pre-existing highs and their intervening depressions. Generally, shallow marine shales, sandstones and limestones were deposited over the region with deeper marine sediments deposited to the south.

A number of the Paleogene fault zones were reactivated during the Neogene, mainly as comparatively gentle flexures which were accentuated by minor faults. Wrench faulting, basin inversion or renewed basin subsidence starting in the Late Miocene appears to have continued into Recent times. In addition to imparting a strong northeast-southwest structural grain to the region with associated north-south oriented en-echelon structures, the later tectonic regime has apparently facilitated local magmatism. Increased heat flow during tectonic activity caused parts of the Paleogene source kitchens to reach maturity before the main pulse of inversion. Where renewed subsidence has occurred, these source rocks are now in the gas window or are overmature.

Structures formed during the Late Miocene are readily identified on seismic data, however they are generally dry because their origin postdates the main phase of hydrocarbon generation. The size of hydrocarbon accumulations in these structures may also be limited by a lack of effective seal. Pre-Late Miocene structures, on the other hand, despite also having suffered a later inversion phase, are an attractive exploration target due to their existence before oil generation.

Potential traps for hydrocarbon accumulation are extremely varied. Along the Madura-Kangean trend in the Early Tertiary basins, Oligocene to Early Miocene pinnacle reefs are surrounded and overlain by deep water shales. The Middle Miocene inversion led to the deposition of turbidite sandstones in some basins, with vertical migration of hydrocarbons through rejuvenated faults. On the shelfal areas, patch reefs and shallow marine sandstones of Oligocene to Middle Miocene age are the targets for exploration. Transgressive basal marine sandstones occur over regional palaeohighs. Reefal limestones of Pliocene age are also of interest where they are sealed by Recent mudstones.