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Abstract

Detailed geologic studies of hydrocarbon reservoirs in platform carbonates commonly show that reservoir zones occur within 1–15-m-thick, upward-coarsening successions of lithofacies, i.e., within upward-shallowing cycles. Our understanding of the depositional history and reservoir characteristics of such cycles and their component facies is enhanced by observations of ancient outcrop examples and models derived from modern analogs.

Outcrops of the Permian San Andres Formation along the Algerita Escarpment of the Guadalupe Mountains contain cycles 3–12 m thick, with thin mudstone/wackestone bases, overlain by burrowed wackestones and packstones, and capped by thick massive to planar or cross-bedded packstones and grainstones. These facies formed during relative rise and/or stabilization of sea level during which carbonate sand shoals developed. The outcrops also display lateral facies relationships within the cycles on the scale of hundreds of meters that are representative of those commonly observed in analogous hydrocarbon reservoirs of the Permian basin of Texas and New Mexico.

Core and surface sediment mapping in the Holocene Joulters Cays ooidshoal complex of Great Bahama Bank reveals the three-dimensional complexity of an upward-coarsening and shallowing cycle. This facies mosaic is like that observed in two dimensions at Algerita Escarpment or in one dimension in a core from a reservoir. This modern example points out difficulties in interpretation and correlation of grainstone cycles in subsurface studies of platform carbonate reservoirs. The modern shoal complex, which extends over 400 km2, varies greatly in thickness but averages 4 m thick. Shoal growth, largely in a response to a relative rise of sea level, records rapid expansion of ooid sands, island formation and associated meteoric diagenesis, local shoal stabilization and reworking by burrowing, and generation of hardground layers.

Sand generation and topography varied greatly in the Joulters Cays area during flooding of the platform and development of the shoal. Such variation should be expected in ancient examples, as was observed at Algerita Escarpment. Within the upper grain-dominated part of the cycle at Joulters, depositional facies geometries and early diagenetic alteration contribute to fine-scale heterogeneities. This is at a scale equivalent to documented hydrocarbon reservoir heterogeneities.

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