October, 1999 HGS Meetings Calendar

October, 1999HGS Meetings

HGS Dinner Meeting
"New exploration plays for Edwards and Sligo Cretaceous margins: untested opportunities in onshore Texas"

Authors: by Dale A. Fritz, Santa Fe Snyder Corporation, Houston, TX,
Date: Monday, October 11, 1999
Place: Westchase Hilton, 9999 Westheimer
Time: 5:30 pm Social 6:30 pm Dinner

Abstract:
We examine the petroleum potential in onshore Texas of the most prolific reservoirs found to date in the Gulf of Mexico Basin, Cretaceous carbonates, particularly the Edwards and Sligo Formations. Combining 2-D and 3-D seismic data with lithologic and biostratigraphic information enabled the generation of a detailed sequence stratigraphic framework that led to a concentrated effort in Lavaca County, a redefinition of the Edwards shelf margin, and confirmation of a major sequence boundary in the Sligo Formation. Extension of the favorable stratigraphy of the Edwards Formation beyond the commonly recognized margin presents abundant opportunities for future exploration in the Edwards. Our work also shows the Sligo Formation to be a well-defined aggradational margin that underwent a major period of exposure, resulting in deposition of a series of downslope debris wedges. To our knowledge, the deposits have not been drilled in the United States and present exciting exploratory possibilities.
Introduction
Our analysis of exploration concepts focused on the Early Cretaceous Edwards and Sligo margins in an area of east central Texas proximal to the San Marcos Arch (Figure 1a). The area was chosen for its structural configuration, which could potentially focus petroleum. Margins of the Edwards are coincident in a few areas but diverge repeatedly along the Sligo trend (Figure 1b). Although the Edwards margin is commonly mapped as coincident with that of the Sligo in the study area, our work clearly demonstrates that the margin actually progrades seaward (southeast) 3 or more miles (>5 km) beyond the Sligo margin.
An interpreted 2-D seismic line illustrates the architecture of the Cretaceous shelf and margin in the study area (Figure 2). Nine sequences occur in the interval from the Cotton Valley Formation to the top of the Edwards. The sequence boundary at the top of our Sequence 5 in the upper Sligo Formation is equivalent to the 112 MY sequence boundary of Goldhammer et al. (1991) and marks a key period of erosion and possible deposition of coarse debris downslope. The boundary between Sequences 6 (uppermost Sligo) and 7 (Pearsall) is interpreted to be a drowning unconformity. At the top of the Edwards, the Sequence 9 boundary coincides with the 98 MY sequence boundary of Goldhammer et al. (1991) and is interpreted to be a time of dissolution of coarse backreef grainstones.
The Edwards - An Underdrilled Opportunity
An arbitrary line through a 3-D seismic volume and a corresponding geologic cross-section show the progradational nature of the Edwards Formation (Figures 3, 4). Three wells are included (Figure 3): the Mobil Kahanek #1 (Bebout and Kupecz, 1985) is projected 13 miles (21 km) along depositional strike from Word Field, while the Chevron Coby #1 and Exxon Joe Zaruba #1 are on the arbitrary line. Sonic or density logs and synthetic seismic traces tied the wells to the seismic data. Four seismic reflectors, labeled 1-4, are interpreted as sequence boundaries from stratal geometries. Three Edwards sequences defined by the four boundaries occur within Sequence 9 on the 2-D seismic line (Figure 2).
Four key seismic reflectors in the Edwards Fm. (Figure 3) were used to assist in correlating the wells on the cross-section (Figure 4). Reflector 1 is a high-amplitude event downdip that diminishes in strength updip (Figure 3). It occurs at the top of a section of deeper water argillaceous wackestones (Upper Tamaulipas) and is equivalent to the sequence boundary '' at the top of Sequence 8 (Figure 2). Reflector 1 is immediately overlain by a prograding reef and bank complex of the Edwards margin as observed in cores from the Kahanek well and distal slope wackestones described from cuttings of the Coby well (Figure 4).
Reflector 2 (Figure 3) is a weak event that ties lagoonal packstone/grainstones in the Kahanek cores to a reef and bank complex in the Coby cuttings (Figure 4). We infer that this reflector ties to forereef and slope deposits in the Zaruba well.
Reflector 3 (Figure 3) ties backreef wackestone/packstones of the Kahanek cores to reef and backreef grainstones in the Coby cuttings (Figure 4). We interpret from the logs of the Zaruba well that a reef and minor forereef succession occur in the interval.
Reflector 4 (Figure 3) ties backreef deposits in the Kahanek well (inferred from logs and observed in the equivalent interval in other Word Field cores) to backreef packstone/wackestones in cuttings from the Coby well. The same deposits are correlated to reef and backreef grainstones in cores from the Zaruba well. Reflector 4 is equivalent to the sequence boundary at the top of Sequence 9 (Figure 2). Clearly, the top Edwards interval between reflectors 3 and 4 represents a progradational package that ultimately culminates seaward of the Zaruba well. Correlations on figures 3 and 4 delineate prospective grainstone packages 3 miles (4.8 km) or more seaward of the published Edwards margin.
The impact of our findings is considerable. We have defined a virtually unprospected area of the Edwards with significant potential for new gas reserves. The highly progradational nature of the Edwards margin places prospective backreef and reef grainstones well seaward of the recognized margin. Comparison of the 2-D seismic expression of Edwards Sequence 9 (Figure 2) with the three Edwards sequences identified on the 3-D data (Figure 3) allowed us to develop a high-resolution sequence stratigraphic framework that reveals the location of favorable facies. Combining these data with the likelihood that faulting (Figure 3) can create avenues for development of secondary porosity in the Edwards and charge the system with petroleum from deeper source rocks (Fritz et al., in press), it is evident that numerous opportunities for exploratory drilling exist. In Lavaca County alone, the new fairway of opportunity is over 3 miles (4.8 km) wide and 25 miles (40 km) long.
Sligo Forereef — An Untested Opportunity
A Sligo debris play is based on a sequence boundary recognized in the upper part of the Sligo in outcrops in Mexico (Goldhammer et al., 1991). We interpret the same sequence boundary on seismic data and recognize a wedge geometry downslope from the Sligo margin (Fritz et al., in press). Base-level change about the sequence boundary would have caused coarse carbonate debris- and grain-flow deposition seaward of the Sligo shelf margin. Data from analogous reservoirs confirm that downslope carbonates can retain reservoir-quality porosity, e.g., Poza Rica Field in east-central Mexico (Figure 1a) described by Enos (1985). Facies variation and slump-faulting on the foreslope create potential for trapping in proximity to deepwater carbonates, setting up a petroleum source and migration pathway.
Figure 5 is a 3-D seismic line illustrating the major sequence boundary in the Sligo margin. The same boundary is identified on 2-D (sequence boundary at top of Sequence 5, Figure 2) and 3-D seismic lines (Figure 3). Several events on the seaward side of the margin have onlap and downlap reflector terminations (Figure 5) and display the proper architecture for part of a downslope debris wedge in excess of 1000 ft (300 m) thick. The Sligo debris wedge (Figure 5) is also visible on the 2-D data (Figure 3; downslope portion of sequence 6).
Existence of a Sligo downslope wedge between the sequence b