FRACTURED GRANITIC BASEMENT AS RESERVOIR

PROCEEDINGS JOINT CONVENTION BALI 2007
The 32^nd HAGI, The 36^th IAGI, and The 29^th IATMI Annual Conference and Exhibition

FRACTURED GRANITIC BASEMENT AS RESERVOIR

H.D. Tjia

Lestari (Institute for the Environment and Development) Universiti Kebangsaan Malaysia, 43600 Bangi, Malaysia

ABSTRACT

Granitic pre-Tertiary basement in northern Sundaland and Indosinia provides two principal reservoir types: fractured competent rock and various forms of its weathered rock. Due to its great dimensions, fractured granite basement blocks of less than 3 per cent bulk porosity have been estimated to individually host over hundred million BBOE. Localised fracture densities and fracture connectivity appear as major constraints to realising the full potential of such reservoirs. Weathered granitic rock has positive reservoir capabilties if it was exposed to semi-arid to arid conditions. Such an environment prevailed at the transition of the Mesozoic to the Cenozoic when northern Sundaland became a vast peneplain. Feldspars and ferromagnesian constituents have not been severely altered and are unlikely to have filled pore space and blocked pore throats. Late Cretaceous granitic rocks outcropping in Peninsular Malaysia and in southern Vietnam were weathered into grus forming metres-thick but kilometre-extensive pediplains, talus, and in-place blankets of partly weathered rock covering flatter surfaces of the exposed plutons. During the Cenozoic, equivalent granitic plutons beneath the currently offshore basins became covered by sediments. Up to Mid-Miocene, block faulting produced horst-and-graben basement structure. The basement highs of fractured and weathered granitic rock became potential recipients of hydrocarbons generated in the topographic lows. Reservoir models of fractured competent granitic rock and of its semi­arid weathered parts summarise decades of study by the author.