Volker C. Vahrenkamp

Growth Architecture, Faulting, and Karstification of a Middle Miocene Carbonate Platform, Luconia Province, Offshore Sarawak, Malaysia

The Mega Platform is a 30- 50-km-large and 1.2-km-thick middle Miocene carbonate platform located in the Luconia Province, offshore Sarawak, Borneo. The platform originated in the late early to early middle Miocene on a regional fault-bounded structural high, first aggraded and then backstepped during a series of third-order sea level fluctuations during the middle Miocene (TB2.32.6). The Jintan Platform termination with an area of 8 12 km is one of the prominent backsteps toward the top of the Mega Platform. Three-dimensional (3-D) seismic indicates that growth on Jintan ceased relatively early with continued carbonate aggradation in adjacent smaller terminations (M1, M1-East). Spectacular reservoir architecture and diagenesis are revealed by the seismic. Several transgressive, aggradational, and progradational cycles are overprinted by repeated karst events. Dissolution features and bank-margin collapse are aligned to a deep-seated regional fault system, which periodically became reactivated during carbonate growth. A large triangular-shaped graben formed during one of the faulting periods but subsequently healed by a prograding reef-margin sequences. Two alternative scenarios are presented to explain the ultimate demise of the platform. The first proposes drowning resulting from a combination of subsidence and eustatic sea level rise. The second evokes a much-later drowning, which was preceded by a long period of exposure resulting from a second-order sea level fall and an initial decrease in subsidence caused by the onset of tectonism in Borneo during the late Miocene. In any case, following a hiatus of about 5 m.y., the platform was finally buried by deep-marine siliciclastics that prograded into the basin from the large delta systems of northwest Borneo. Recognition of growth architecture, faulting, and karstification is a key to exploiting the hydrocarbon reservoirs of the Mega Platform. A 30-m-thick low-porosity and -permeability layer shields the gas trapped in Jintan from the underlying aquifer. Penetrated by only one well, the extent of the layer and areas of breaching caused by faulting and karstification are identified on seismic. Interpretation of the seismic is critical to assessing whether and how the underlying aquifer is felt during reservoir depletion and whether there is pressure communication between adjacent reservoirs connected via the aquifer. Cores and logs from three wells provide ground truthing of reservoir architecture, karst features, and faulting derived from the interpretation of reflection and inversion seismic. The interpretation is then imported into static and dynamic 3-D models to constrain reservoir properties, predict dynamic behavior, and guide optimum field development.

Supraglacial Diamictons at Continental Ice Margins: Pleistocene Alluvial Fan-Flow Till Deposition in Southeastern Michigan: ABSTRACT

Glacial sediments more than 50 m (165 ft) thick were laid down over southeastern Michigan during retreat of the late Wisconsinan continental ice sheet. As a result, the topography of the region is now dominated by recessional moraines composed of thick sequences of outwash sand and gravel, which are in turn capped by up to 7 m (23 ft) of glacial diamicton. Sedimentary structures exposed in many gravel pits in the Fort Wayne moraine suggest that outwash sequences were deposited as proglacial alluvial fan systems which were partially overridden during short periods of ice readvance. Several features, including abruptly truncated trough cross-bedding in gravels and truncated large clasts of previously ice-cemented sand at outwash-till contacts, require shearing at outwash su faces either prior to or during till emplacement. Such outwash till contacts suggest that some of the diamicton was deposited in subglacial settings as lodgement till. Other exposures, however, exhibit gradational outwash till contacts, fluidized mixtures of thin outwash and till layers, and till draped over large sand clasts. Such features require that much of the diamicton was emplaced without truncation at the outwash surface, and suggest that deposition occurred in supraglacial settings as flow till. The distribution of lodgement till and flow till in this region indicates that lodgement tills predominate on proximal (iceward) portions of moraine slopes and that flow tills predominate on distal slopes. This distribution suggests that during outwash till deposition, the front of the continental ice sheet had readvanced only to the moraine crest, and that a single depositional episode gave rise to the entire outwash till sequence. Similar features are typical of other moraines in the region and suggest that, in general, the crests of gravel-cored Pleistocene moraines coincide with the maximum limits of ice readvance and delineate areas of lodgement till deposition on proximal slopes and areas of flow till deposition on distal slopes. End_of_Article - Last_Page 563------------