Sea Level Fluctuations and Ooid Diagenesis in the Round Top Palo Pinto Field, Fisher County, TX

MITCHELL, RAYMOND W., Conoco Inc., Ponca City, OK, and JOHN COSS, Conoco Inc., Midland, TX

The principal reservoir facies in the Missourian Round Top Palo Pinto field are ooid grainstones and skeletal packstones/wackestones which cap shallowing-upward depositional cycles. Individual cycles form flat-bedded, tabular limestones 10-50 ft thick, and at least 15 cycles are identified in the 650-ft-thick interval. Individual cycles are laterally extensive and correlatable throughout most of the field. The cycles exhibit abrupt erosional truncation at the contact with an onlapping Virgilian-age clastic sequence, and intraformational erosional scarps with relief up to 150 ft are present.

A complete cycle is comprised of basal black shale/mudstone/wackestone, overlain by crinoid/foram wackestone/packstone, capped by ooid/skeletal grainstone. The composition and morphology of the cycles are the result of the interaction between subsidence, carbonate production rates, and sea level fluctuations.

The ooid grainstones are 5-20 ft thick and characterized by oomoldic porosity and intergranular spar cement (porosity inversion). The uppermost 2-5 ft of the grainstones are commonly tightly cemented. The ooid grainstones formed during sea level highstands as carbonate production exceeded subsidence, resulting in aggradation. During subsequent lowstands, freshwater lenses were established on exposed oolite islands. Calcite spar cement filled interparticle space and partially replace ooid cortices. Replacement and dissolution fabrics in the ooids are similar to those reported from freshwater phreatic lenses of modern oolite islands in the Bahamas.

The fabrics observed in these modern and ancient oolites are common in other ancient ooid grainstones. Early diagenetic alteration in freshwater lenses may have been a common process in ancient ooid grainstones.

AAPG Search and Discovery Article #91008©1991 AAPG Mid-Continent Section Meeting, Kansas Geological Society, Wichita Kansas, September 22-24, 1991 (2009)