The Holocene Sequence--Implications for Correlating Parasequences and Reservoir Layers

HARRIS, P. M., Chevron Oil Field Research Company, La Habra, CA, D. G. BEBOUT, and C. KERANS, Bureau of Economic Geology, University of Texas, Austin, TX

A successful geological approach to development of hydrocarbon reservoirs in carbonates begins with the detailed description of continuous cores from wells that are widely spaced throughout the reservoir. After calibrating downhole logs from uncored wells, the thinnest geologically meaningful sequences of facies are chosen, mapped field-wide, and then assigned petrophysical and production properties. Commonly, these reservoir zones are an upward-shoaling sequence of carbonate facies, i.e., shoaling cycle or parasequence.

Our understanding of the deposition of such sequences and component facies is enhanced by observations and models derived from modern analogs. For example, the coring of modern sediments in the Joulters Cays area of Great Bahama Bank has revealed the complexity of the modern sequence. Covering 400 sq km and averaging 4 m thick, the sequence graphically shows the rapidity with which complex facies relations can form and points out the difficulties of interpretation and correlation of cores from ancient examples. The Joulters shoal formed in only the last 5000 years. Nevertheless, the facies record of shoal growth, largely a response to a relative rise in sea level, indicates significant changes such as rapid expansion of ooid sands, island formation and associated meteoric diagenesis, ajor reworking by burrowing, and generation of hardground layers. In some facies the depositional texture, sedimentary structures, and grain types have all been modified since deposition. The shoal was more active in the recent past over a larger area than today, and ooid packstone, the most widespread facies, represents reworking of

grainstones through burrowing during development of a vast sand flat.

In San Andres and Grayburg reservoirs of the Permian basin, the fundamental sequence, 3-12 m thick, often contains a thin mudstone base overlain by burrowed wackestone-packstone and capped by a thick planar- to cross-bedded packstone-grainstone. These facies, like the modern example, formed during a relative rise in sea level with the accompanying development of oolitic, peloidal, and skeletal shoals. Outcrop examples from the Algerita Escarpment of the Guadalupe Mountains and subsurface correlations from Farmer field show lateral facies relations within a sequence that are surprisingly similar to those of the modern example.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)