Petroleum Geoscience - Volume 15, Issue 2, 2009

The offshore Indus Basin is a rift and passive margin basin offshore Pakistan and northwest India. Rifting associated with break up of the India/Madagascar/Seychelles plates began during the Late Cretaceous and was accompanied by a major period of volcanism associated with the Deccan volcanic event. A major volcanic centre was located along the south of the basin adjacent to the Saurashtra Arch transform fault and resulted in the deposition of up to 7 km of extrusive basalts interbedded with Late Cretaceous–Paleocene marine sediments. The basalts show stacked prograding reflection patterns on seismic data. A chain of northeast–southwest-trending volcanic seamounts in the central part of the deep-water basin formed topographic highs for the development of shallow-water carbonates during the Eocene post-rift phase. The passive margin stratigraphy includes up to 9 km of Oligocene–Recent age clastic sediments from the Indus River system. The proximal part of the Indus Fan contains spectacular large-scale channel-levees in the Miocene and Plio-Pleistocene intervals. Fourteen major channel-levee systems have been identified in the Plio-Pleistocene and represent potential reservoir targets. Trap types include extensional rollover anticlines at the shelf edge, drape structures over the Eocene carbonate highs, stratigraphic traps along the Murray Ridge and folds associated with strike-slip faults along the Murray Ridge. A key challenge for future exploration is to determine whether source rocks are present in sufficient quality for commercial discoveries.

With the Mangala oil discovery in 2004, Cairn established the Barmer Basin of Rajasthan as a major new hydrocarbon province. Most reserves are contained in fluvial sandstone reservoirs of the Fatehgarh Formation, which probably ranges in age from Late Cretaceous to Early Paleocene. The Fatehgarh sandstones were mainly derived from reworking of Mesozoic sandstones at the northern end of the Barmer rift, but with some volcaniclastic input probably derived from Deccan volcanic rocks within and on the margins of the rift. These thick, quartz-rich, high porosity and permeability sandstones provide an excellent oil reservoir in the north of the Barmer Basin, but the increasing volcanic influence further south causes reservoir quality and thickness of net sand to deteriorate. This paper relates how the tectonic and volcanic evolution of the northwest margin of the Indian plate has influenced the depositional trends which have resulted in formation of this world class reservoir.

There are numerous hydrocarbon-rich sedimentary basins in Indonesia, Malaysia and southern Thailand. Almost all these basins began to form in the Early Cenozoic, they are filled with Cenozoic sediments, most are rifted basins that are the product of regional extension, and they formed mainly on continental crust. Many different models have been proposed to account for their formation and age. Understanding basin development requires a better knowledge of the Mesozoic and Early Cenozoic history of Sundaland, which mainly lacks rocks of this age. Sundaland is a heterogeneous region, assembled from different continental blocks separated by oceanic sutures, in which there has been significant Mesozoic and Cenozoic deformation. It is not a ‘shield’ or ‘craton’. Beneath Sundaland there is a marked difference between the deep mantle structure west and east of about 100°E reflecting different Mesozoic and Cenozoic subduction histories. To the west are several linear high velocity seismic anomalies interpreted as subducted Tethyan oceans, whereas to the east is a broad elliptical anomaly beneath SE Asia indicating a completely different history of subduction. Throughout most of the Cretaceous there was subduction north of India, preceding collision with Asia. However, north of Australia the situation was different. Cretaceous collisions contributed to elevation of much of Sundaland. Subduction beneath Sundaland ceased in the Cretaceous after collision of Gondwana continental fragments and, during the Late Mesozoic and Paleocene, there was a passive margin surrounding most of Sundaland. When Australia began to move northwards from about 45 Ma, subduction resumed at the Sunda Trench. Basins began to form as the region went into compression at the time of subduction initiation. There was widespread extension, broadly orthogonal to the maximum compressive stress but modified by pre-existing basement structure. After subduction resumed, the weakness of the Sundaland lithosphere, unusually responsive to changing forces at the plate edges, meant that the basins record a complex tectonic history.

It has been accepted for many years that eastern Borneo and western Sulawesi were close together in the Late Cretaceous but the mechanism and age of formation of the Makassar Straits, which now separate them, have been the subjects of much debate. Geological studies on land show that the straits formed by Eocene rifting. However, the nature of the crust beneath the straits remains controversial. The southern parts are likely to be underlain by extended continental crust but, in the northern Makassar Straits, it is more difficult to decide. Water depths are up to 2500 m, there is a very thick sedimentary cover, the basement is not well imaged on seismic lines and there is no way of directly sampling it. Field studies from the Borneo and Sulawesi margins have provided the basis for reconstructing the development of the straits, and suggesting they are underlain by oceanic crust. The rift and its margins are asymmetrical and wide, with up to 400 km of stretched crust on the Borneo side and about 200 km on the Sulawesi side, separated by about 200 km of the deepest crust in the northern Makassar Straits. Gravity data and flexural modelling on the Borneo side suggest a junction between continental and oceanic crust beneath the Mahakam delta. The oceanic crust is inferred to be of Middle Eocene age, similar to the Celebes Sea to the north; apparent conical structures on seismic lines have been interpreted as volcanic edifices. However, the earliest backstripping studies suggested thinned continental crust in the central straits and this has been supported by interpretations of new seismic data from the offshore area west of Sulawesi. Half-graben and graben are interpreted beneath thick sediments, there are low-angle extensional faults, and lineaments crossing basement can be traced into the deepest parts of the straits. These structures suggest an origin by oblique rifting of continental crust in which the apparent conical structures are interpreted as carbonate build-ups on tilted fault blocks.

Java is part of a volcanic island arc situated in the Indonesian archipelago at the southern margin of the Eurasian Plate. Sundaland continental crust, accreted to Eurasia by the Early Mesozoic, now underlies the shallow seas to the north of Java where there has been considerable petroleum exploration. Java has an apparently simple structure in which the east–west physiographic zones identified by van Bemmelen broadly correspond to structural zones. In the north there is the margin of the Sunda Shelf and, in southern Java, there are Cenozoic volcanic arc rocks produced by spatially and temporally discrete episodes of subduction-related volcanism. Between the Sunda Shelf and the volcanic rocks are Cenozoic depocentres of different ages containing sedimentary and volcanic material derived from north and south. This simplicity is complicated by structures inherited from the oldest period of subduction identified beneath Java, in the Cretaceous, by extension related to development of the volcanic arcs, by extension related to development of the Makassar Straits, by late Cenozoic contraction, and by cross-arc extensional faults which are active today. Based on field observations in different parts of Java, we suggest that major thrusting in southern Java has been overlooked. The thrusting has displaced some of the Early Cenozoic volcanic arc rocks northwards by 50 km or more. We suggest Java can be separated into three distinct structural sectors that broadly correspond to the regions of West, Central and East Java. Central Java displays the deepest structural levels of a series of north-directed thrusts, and Cretaceous basement is exposed; the overthrust volcanic arc has been largely removed by erosion. In West and East Java the overthrust volcanic arc is still preserved. In West Java the arc is now thrust onto the shelf sequences that formed on the Sundaland continental margin. In East Java the volcanic arc is thrust onto a thick volcanic/sedimentary sequence formed north of the arc in a flexural basin due largely to volcanic arc loading. All the components required for a petroleum system are present. This hypothesis is yet to be tested by seismic studies and drilling, but, if correct, there may be unexplored petroleum systems in south Java that are worth investigating.

In 2000, a large marine controlled-source electromagnetic (CSEM) calibration survey was acquired by Statoil as a part of a CSEM evaluation project over known hydrocarbon (HC) reservoirs offshore Angola. The electric field magnitudes measured at the seafloor above the HC reservoirs are 1.5 to 3 times higher compared to the synthetic background responses and ‘off-reservoir’ measurements. The relative strength of the electric field measurement above a salt structure is very strong (normalized magnitudes stronger than 3) compared to the background response. Modelled responses obtained from three-dimensional finite difference time-domain forward modelling, based on seismic and petrophysical data, show good agreement to the measured responses over the HC reservoirs. The modelled response after introducing a shallow resistor to the 3D model explains the strong CSEM anomaly measured in the NE part of the study area with no proven HC reservoir. The 3D forward modelling is also used to predict the sensitivity to the resistivity of the undrilled structure in the SE part of the study area, where no CSEM data were acquired. Integration of structural information interpreted from seismic data and resistivity values estimated from well logs significantly improves the subsurface geo-resistivity modelling and complements the offshore Angola CSEM data interpretation.