DEPOSITIONAL SYSTEMS IN THE OLMOS FORMATION AND THEIR RELATIONSHIP TO VOLCANIC HIGHS AND HYDROCARBON EMPLACEMENTS, ZAVALA COUNTY, TEXAS
Authors: INDEST DANIEL J. , McPHERSON JOHN G. 
Published: October 2019

Late Cretaceous volcanics, in the form of small, isolated bodies of submarine extrusives, are common in Central and South Texas. Hydrocarbon traps associated with these volcanic mounds have been known for some time. This is a detailed case history of one locality (Zavala County) where the Upper Cretaceous Olmos Formation (Taylor Group) has been locally affected by over 30 individual volcanic bodies.

The Olmos Formation crops out in Maverick County, Texas, and dips southeastward into the subsurface of Maverick, Zavala, Dimmit, Webb, Frio, LaSalle, and Atascosa counties. The texture, composition, and detailed stratigraphic relationships of the subsurface Olmos Formation of Zavala County were determined from the analysis of 342 electric logs and core samples from numerous wells in the county. The analysis established that the Olmos Formation was derived from highlands of the Laramide Uplift to the west and deposited by fluvio-deltaic systems in the easternmost part of the Rio Grande Embayment of the Gulf Coast Basin.

The Olmos has been subdivided into six lithofacies (A to F, oldest to youngest) on the basis of the shape of electric log curves, grain size relationships, and petrographic data. The geometry of these lithofacies was clearly delineated by net-sandstone maps. Lithofacies A is a thin coarsening-upward sequence which is interpreted as a small fluvial-to wave-dominated deltaic deposit. Lithofacies B, a coarsening-upward sequence with minor mudstone breaks, was deposited as a nearshore bar sand. Lithofacies C is a coarsening-upward sand overlying an eastward-thickening mudstone and represents a large eastward prograding, fluvial-dominated delta. Lithofacies D is a coarsening-upward deltaic sequence similar to Lithofacies C but with a shift in the depositional axis. Fining-upward point-bar sandstones, mudstones, and coals of Lithofacies E are the laterally equivalent delta plain deposits of A, B, C, and D. Lithofacies F is a thin, transgressive sandstone formed by reworking as the underlying progradational system foundered and the shoreline moved westward.

A detailed study of the net-sandstone isopachs of the Olmos Formation showed that there is a well-defined correlation between the isopach lows and the position of underlying Late Cretaceous volcanic mounds. The effect is best shown by Lithofacies C and E. The thinning of the Olmos Formation is thought to have been caused by the rapidly prograding fluvial-dominated Olmos depositional system which caused the thickest sands to be deposited away from volcanic topographic highs. Lower compaction rates over these volcanics undoubtedly contributed to the depositional thinning of the sands.

The stratigraphic relationships produced by the thinning and draping of the Olmos Formation over the volcanics generated early potential traps for hydrocarbons. Continued differential compaction over the volcanics produced localized, generally northeast-southwest trending horsts and grabens over the volcanic highs, creating an additional trapping mechanism. Presently, there is a well-defined relationship between the location of producing wells and fields associated with the volcanic bodies and thinning and faulting of the Olmos Formation in Zavala County.