Carbonate Hardgrounds: Their Use as Stratigraphic Markers and Their Significance in Porosity Development in the Lower Cretaceous Edwards Limestone, Word Field Complex, Lavaca County, Texas

MOORE, S. P., Mobil Exploration and Producing U.S. Inc., Houston, TX, and S. E. AKTURK, Mobil Research and Development Corporation, Dallas, TX

Edwards gas production at the Word field complex is from back-reef carbonate environments associated with a broad regional arch and is anomalous along the Cretaceous shelf margin. Over 3000 ft of core were used to determine the factors controlling porosity development and to calibrate electric log responses for reservoir zonations. The depositional facies in the cores include open marine, lagoon, and island paleoenvironments. The island complex (which includes beaches, tidal flats, and tidal channels) controls the distribution of the production. Numerous episodes of syndepositional exposure are recognized in the cores. The most porous zones in the sequence are below the exposure surfaces in the grainstone shoal deposits of the beach environment. The evolution and identification of the e exposure surfaces is critical to understanding reservoir quality and to subdividing and correlating the Edwards into hydraulic units.

Hardgrounds in the Edwards evolve during both subaerial and submarine exposure. The leaching and cementing processes below these exposure surfaces have a strong control in the distribution of porosity and permeability. The dominant porosity type is intraparticle microporosity, formed preferentially in micritic or micritized grains and in the patches of interparticle lime mud. Some microporosity later converted into moldic porosity due to leaching. The influence of these exposure surfaces on the porosity trends is significant in determining the connectivity of the reservoir.

The original fabric of each lithofacies is modified by the exposure processes. Three types of exposure surfaces are recognized in the cores by their structure, color, and texture: (1) submarine hardgrounds characterized by encrustations, borings, vertical burrows, and marine cements; (2) subaerial exposure surfaces characterized by caliche crusts and pisolites, dessication features, vadose cement, brecciation, dissolution vugs and channels, and multiple episodes of infilling; and (3) pedolithic surfaces (related to prolonged postdepositional emergent conditions) characterized by clay paleosols with caliche nodules overlying rooted caliche crusts and occasionally red sediment-filled fissures.

The hardgrounds in the Edwards Limestone are widely traceable in the Word field complex and form an integral part of the porosity development cycles. The correlation of the porosity trends in the cores as delineated by hardgrounds corresponds to reservoir units interpreted on electric logs. The sonic porosity logs show close agreement with core measured and visual porosity estimates, while the gamma log responds to lithofacies changes. The consistent physical recognition of these hardgrounds and their stratigraphic relationship to porosity trends permits their use as field-wide stratigraphic markers. The carbonate hardgrounds define hydraulically connected porosity zones within the Edwards reservoir and, therefore, can be used to better define the in-place reserves.

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)