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Porosity Evolution in the Batu Raja Carbonates of the Sunda Basin — Windows of Opportunity
Much of the effective porosity within the productive carbonates of the Batu Raja Formation has a meteoric fresh water origin. Pervasive neomorphic alteration of the original textures, the prevalence of radial fibrous cements, and de-dolomite are among the petrographic features that support this argument. Sunda Basin palaeogeography and basin evolution suggest that the timing for this porosity generation was early.
Hot cathode luminescence, electron microprobe and isotopic analyses supplement basic petrographic studies and prove invaluable in resolving the paragenesis and timing of porosity development. Results indicate that there have been six phases of cementation interrupted by two phases of dissolution and porosity enhancement, of which the earlier is the more important.
Burial history of the Sunda Basin, throughout the Miocene, is one of progressive subsidence, yet the cyclic patterns evident on electric logs coupled with the petrographic and isotope data imply that during its early history this subsidence was episodic. Rates of carbonate dissolution under tropical conditions are not only exceptionally rapid but can also vary locally. Nevertheless, the enhanced pore throat sizes so created, even when small, can be extremely effective conduits capable of delivering producible hydrocarbons at very high flow rates. Zones of only a few feet thickness are potentially viable pay intervals and, thus, become attractive. It is suggested that hiatuses resulting from 4th and 5th order cycles of sea level change, within the early Miocene, provided ready opportunities to both create and preserve the cavernous porosity and associated multi-darcy permeabilities that allow for locally prolific hydrocarbon production.
While the brief time spans available preclude any major modification of the paleo-surface relief, rendering it all but invisible to seismic, certain amplitude and reflector anomalies are attributable to karst activity. These are now being actively pursued as infill and exploration drill targets within the Lower Miocene carbonates of the Sunda Basin. The rocks hold the key to the recognition of karstic features and identification of attendant porosity development. When combined with basin evolution and migration studies, the rock data become a powerful exploration and development tool.
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