Production of gas and some condensate from fine-grained fractured sandstone of the Upper Cretaceous Woodbine-Eagle Ford interval at depths of 10,800 to 11,350 ft in central northern Tyler County has provided the impetus for a detailed paleoenviron-mental analysis of the geology in that area. The productive area (Sugar Creek field) is located a short distance south of the Sabine uplift, which was an active positive area previous to, during and following Woodbine-Eagle Ford deposition, and is slightly down-dip from the Lower Cretaceous continental shelf edge as delineated by the Angelina-Caldwell flexure and the Edwards reef trend. The Woodbine-Eagle Ford interval (between the Buda Limestone below and Austin Chalk above) is 150-200 ft thick in the Sugar Creek field area but thins to less than 50 ft thick above the Edwards reef buildup and northward toward the Sabine uplift where it is missing. Southward (down-dip) the interval thickens to greater than 1500 ft within a distance of 15 miles. The Woodbine-Eagle Ford interval in this down-dip position is a mud-dominated clastic wedge. Cores from seven wells in the Sugar Creek field and two down-dip wells were examined in detail. Dark gray, organic-rich, silty shale with thin laminated to ripple-bedded siltstone beds and small siderite nodules comprise most (40% to greater than 80%) of the Woodbine-Eagle Ford interval and contain a microfauna (foraminifera) indicative of outer shelf to upper slope water depth. The reservoir sandstones occur as complex, single to multi-story bodies 15-40 ft thick and are composed of fine- to very fine-grained quartz arenites. As viewed in polished core slabs, the sandstones are mostly “massive-appearing” (without discernible sedimentary structures). Beds are characterized by very sharp (non-gradational) basal contacts (sandstone/shale) with abundant drag marks, flute casts and other sole markings, and by abrupt upper contacts with shale. X-ray radiography of core slabs has revealed a multitude of sedimentary structures in the otherwise “massive” sandstones. Massive to laminated and cross-stratified sandstone is dominate, but ripple-stratification, soft-sediment-deformation and scour features are also present. Burrows and bioturbation are common but confined only to the upper parts of sandstone beds which may be separated by thin (1-2 inch) shale beds. These sedimentary features and their positions within well-defined sandstone genetic units indicate rapid deposition of sand by low- to high-concentration submarine density (turbidity) currents and associated tractive currents. Mud deposition and burrowing of the upper parts of sand beds occurred during quiet periods between the sand pulses. Highly deformed siltstone intervals often are present below the sandstone bodies and indicate rapid loading by sand deposition and/or slumping on unstable slopes. A conglomerate submarine debris flow deposit is also well displayed in one core. Subsurface correlation and mapping of the discontinuous, lenticular sandstone bodies indicate that they are best delineated as a series of coalescing, dip-oriented lobes. Deposition appears mostly to have been as prograding submarine fan lobes, with sediment being channeled from up-dip delta and nearshore deposits across a narrow shelf and through shelf-edge breaks and then dumped downwlope. These basin-filling deposits pro-graded seaward until the sediment source was cut off and subsequent deposition of the Austin Chalk occurred. Although a major erosional unconformity exists above the Woodbine to the north, no such unconformity can be documented above the down-dip Woodbine-Eagle Ford interval in Tyler County.
The detailed sedimentological analysis of whole-diameter cores is greatly enhanced by the core being in good condition; ideally, cores should be complete (100% recovery) and well labeled with all pieces in correct order. Considerable information loss occurs when cores are mishandled; therefore, specific procedures should be followed. At the well site care should be exercised to preserve correct orientation of individual core segments and the core should be properly marked. Hügel orientation grooves cut in the core will facilitate core reconstruction. Commercial core analysis laboratories usually generate the greatest amount of information loss; therefore, special care should be used during subsampling for fluid-saturation analyses and porosity-permeability measurements. A core gamma-ray scan should be obtained whenever possible to aid in later core-to-log correlation. Finally, prior to detailed examination, it is recommended that the core be slabbed, lapped and photographed. X-ray radiography may be needed to enhance subtle sedimentary structures in massive-appearing sandstones and mudrocks. For proper sedimentological core analysis we recommend a process-sedimentology approach which emphasizes detailed lithologic description and the recognition of genetic units within the vertical sequence. A continuous, detailed sketch should be made and a description made using a check list of important lithologic features. An understanding of Walther’s Law is required for maximum use of the vertical sequence. At least four types of genetic units can be delineated to interprete the physical, biological and chemical processes responsible for generating the sedimentary rock product of the core. Those types are: sedimentation unit, ichnogenetic unit, soft-sediment-deformation (s-s-d) unit, and diagenetic unit. Finally, the sedimentological core analysis should be used to calibrate the wire-line logs and associated sursurface data. The main steps in such calibration are to first determine the core-to-log depth correction and then to determine the level at which genetic units and lithofacies can be recognized on the logs. Such calibration leads to better correlation of sedimentological information to nearby non-cored wells and allows for lateral extension of predictive sedimentological models throughout the subsurface study area.
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