title: Architecture of an Upper Cretaceous volcanic complex and associated carbonate systems: Elaine field, South Texas
Creators: name: Zeng, Hongliu
name: Loucks, Robert G.
name: Reed, Robert M.
Publisher: name: The University of Texas at Austin
Year: 2024
Subjects: subject: Elaine field, South Texas
subject: Upper Cretaceous
subject: carbonate systems

description: Objectives, scope: In South Texas, Upper Cretaceous volcanic mounds have been producing oil since 1915 from “serpentine plugs”, or reservoirs from “tuff cone” type volcanos. The hydrocarbon reservoirs include porous volcanic tuffs, carbonate units on and around the mounds, and shallower sandstone units. Elaine field in Dimmit and Zavala Counties provides an opportunity to study high-resolution 3D architecture of a tuff-cone-type volcano that has not been reported in the literature. We investigated the effective workflow that integrates core, wireline-log, and seismic data, and the methods to improve stratigraphic correlation and reservoir prediction in and around a volcanic mound. Methods, procedures, process: The 3D-seismic data (~120 km2) have a dominant frequency of 30 Hz in the bandwidth of 10-70 Hz. Calculated vertical resolvable limit (λ/4, λ =dominant wavelength) is 16.7 ms (30 m at 3600 m/s to 50 m at 6000 m/s). Seismic data provide basic information on interpretation of structures and faults. Similarity (coherency) attribute is calculated to highlight faults and lithofacies boundaries. In the study area, six wells have cores associated with the volcanic mound, providing basic and key information of lithofacies and depositional environments. Some deeper wells penetrate the carbonate units and whole length of the volcanic section. Many more shallow wells reach only the top of mound, penetrating at least the carbonate units. Observations from nearby Uvalde County outcrops also provide useful calibration for wireline-log and seismic interpretation. Seismic velocity effects (velocity pull-down and push-up) can be useful in predicting lateral lithofacies variations if the contrast of velocity between thick rock bodies are adequate. Additionally, frequency fusion, a special seismic attribute, can be applied to increase the thickness visualization range to λ/16 to λ (4.2 to 66.7 ms), making it possible to image beds between 13 m and 200 m (for high-velocity rocks at 6000 m/s). The application of stratal slices further improves interpretation of seismic geomorphology by making use of the horizontal resolution power of 3D-seismic data. This investigation suggests that, with rock-physics permitting, seismic sedimentology provides a close approximation of the paleogeography of the volcanic mound and associated carbonate systems. Results, observations, conclusions: 1. Our workflow includes six steps: (1) Core study ; (2) Rock-physics modelling; (3) Mapping of seismic velocity effect; (4) Frequency fusion; (5) Seismic facies analysis; and (6) Geologic interpretation. 2. Seismic facies. In this study, the concept of seismic facies is expanded from its original definition. We promote the use of seismic lithological information and seismic geomorphologic patterns on multiple stratal slices or horizon slices to supplement classical seismic-facies analysis. Recognized seismic facies groups include host-rock seismic facies, volcanic seismic facies, and volcanic-mound carbonate seismic facies; each is composed of several subfacies. 3. Geologic interpretation. Three types of volcanic architectural features are interpreted from core, wireline-log, and seismic facies: (1) Magma-feeder system. Deep-rooted faults in the strata below the volcanos have been recognized as pathways for magma that constructed the volcanic mounds (Figure 1, high-angle black lines in the host rock); some of the faults are filled with basalt dykes (Figure 1, high-angle red lines). On stratal slice S2 (Figure 2a), multiple radial and concentric fault patterns are identified, which are partially filled with basalt dykes. (2) Craters and fills resulted from multiple phreatomagmatic eruptions and fills of fine-grained pyroclastic (ash) sediments (Figure 1, craters and tuff cones). (3) Penecontemporaneous and post-volcanic carbonate sedimentation. Seismic facies around S4 (Figures 1 and 2b) indicate carbonate sedimentation on top of the volcanic mound, including shoal and lagoon, and on the flanks and toe inducing slump and gravity-flow aprons. Significance, novelty: The study reveals a complex magma-feeder system, a large volcanic buildup composed of secondary craters and tuff fills, and shallow carbonate sediments and debris-flow fans. Unique tuff-cone type volcanic process offers a clue to correlate tuff reservoirs for optimized drilling. The key is to understand post-depositional alterations super- imposed on the highly unstable tuff-fill bodies. The tuff-cone deposits are controlled by secondary craters, with an overprint of post-depositional alternation. A series of concentric faults are identified crossing the secondary craters. An improved stratigraphic and reservoir correlation calls for working within the boundary of each secondary crater.