Henry W. DeJong
Consulting Geologist
Littleton, Colo.
A regional study and evaluation of the Kerr basin west of San Antonio and southwest of the Llano uplift not only presents new insights into the geologic history of a potentially productive area but supports the conclusion that oil and gas in commercial quantities await the drill.
Minor production from Ordovician Ellenburger limestone was established in Mountain Home field in 1982 with the drilling of Everest Minerals 1 Robby, TD 3,886 ft. The 31 gravity oil is from perforations at 3,720-30 ft, and the IP was 20 b/d with no water. Oil rate later increased to 43 b/d but has since declined. Cumulative production exceeds 60,000 bbl.
It is interesting that this is a one well field and that attempts to offset production on all sides have failed, although substantial shows have been recorded in the attempted offsets. The fact remains that production, albeit minor, has been established in the area, and this proves the oil productive capacity of Ellenburger.
MULTIZONE POSSIBILITIES
Potential production is not limited to Ellenburger, however. Successful gas completions have been made in Edwards, Kimble, Real, and Sutton counties from Pennsylvanian and Permian horizons with some oil and gas from Ellenburger in Sutton and Edwards counties to the west and northwest.
Several wells in the Sisterdale area of north-central Kendall County encountered Ellenburger shows of 23 gravity oil, but attempts at economic completions failed. Numerous shows of oil and gas have been reported in virtually the entire section from Cambrian upward in Kerr, Bandera, Real, and Edwards Counties. One well in southwest Kimble County had an IP of 87 b/d of 17.5 gravity oil from 30 ft of Ellenburger at about 3,900 ft. Production declined rapidly.
One of the most interesting attempts to date was Gulf 1 Gallant, TD 4,620 ft, located in north-central Bandera County. This well flowed 240-360 b/d of 37 gravity oil for 1 1/2 days from Ellenburger perforations at about 4,600 ft. The operator believed the reservoir, at 4,417-4,598 ft, was depleted during testing.
Trap types possible in the Kerr basin area range from simple anticlines, faulted anticlines, and faults through stratigraphic relationships and erosional highs. The tectonic events of the area have been severe at times, but these events coupled with the resulting sedimentation patterns give rise to a good potential for accumulations in a wide variety of traps.
STRATIGRAPHIC REVIEW
The Precambrian consists of granite, schist, gneiss and marble. This thick sequence of faulted metamorphic rocks with large igneous intrusives served as the platform for subsequent sedimentation (Fig. 1).
Initiation of Paleozoic sedimentation began with the encroachment of Cambrian seas from the south and southwest that overlapped the uneven Precambrian topographic surface that exhibited a fair amount of relief. The maximum thickness of about 1,800 ft was dominantly sand that was derived from the northeast and north largely off the area now occupied by the Llano uplift. Some shale and minor limestone were deposited as well, with sandy limestone becoming more prominent in the upper portion of the section mapped as Cambrian. This sand section could be a good reservoir under proper structural conditions.
Oil and gas shows were encountered in wells in southwest Kendall County and in southeast Kerr and Bandera counties. A well in western Bandera County also had oil shows.
In late Cambrian time sedimentation became dominated by carbonate deposition, and this pattern continued upward through Ordovician time without a hiatus so that late Cambrian and Ellenburger units are not readily separated in the subsurface (Fig. 2). The Upper Cambrian-Ellenburger sequence, which varies in thickness from 750-1,850 ft, consists of limestone, dolomite, chert, and shale and represents deposition on a shallow shelf. It can serve as source, reservoir, and seal. The accompanying map shows the major tectonic features for the area and Ellenburger 1,000 ft and 2,000 ft contours. Significant production, summarized elsewhere, has been found in this unit, and equally significant shows of oil and gas occur throughout the basin.
The best shows and production to date in the Kerr basin are associated with a fractured Ellenburger section, and fractures offer the best opportunity for production. The advent of horizontal drilling techniques holds promise of an increased likelihood of sizable, sustained production from this objective.
ABOVE ELLENBURGER
Unconformably overlying the Upper Cambrian-Ellenburger is a sequence of fossiliferous limestones with some shales that varies in thickness from zero around the south flank of the Llano uplift to 450 ft in the southern part of the area. This unit, herein called Pennsylvanian basal limestone, can serve as both source and reservoir, and also represents deposition in a shallow water shelf environment.
Lithologies consist of tan to buff, fine to coarsely crystalline, finely fossiliferous limestone with thin dark shales and with a cherty detrital zone at the base. Very thin remnants of Silurian, Devonian, and Mississippian sediments may be included in this unit but cannot be readily sub-divided with the data at hand.
Oil and gas shows occur in the area, and at least one gas well with an IP of 1.4 MMcfd was drilled in north central Real County in an area of thick limestone.
The basal limestone exhibits thicks and thins, and there are some areas within the Kerr basin where these sediments are absent, probably due to non-deposition although there may have been some erosion prior to the deposition of a thick clastic section over it. If this latter possibility is true, it may document the first movement of the Ouachita Thrust.
SHALLOWER ZONES
The Permo-Penn clastic sequence has potential for gas accumulations in stratigraphic traps. Regional work highlights that potential; what remains is the need for a detailed analysis of the individual sands to locate traps.
This clastic section consists of Pennsylvanian shales, sandstone, and local carbonates varying in thickness from zero to 1,000 ft. The Permian rocks are conformable on the Pennsylvanian, which in turn is conformable on the Pennsylvanian basal limestone below. These clastics form a depocenter defining the basin.
The series of regional maps discloses some interesting facts about this thick Permo-Pennsylvanian section. The unit thins south and southwest off the Llano uplift and reaches a thickness of more than 12,000 ft in a southerly direction.
The isopach contours reflect the presence of the south-southwest trending Edwards arch in Kimble and Edwards counties. Unconformably overlain by the Cretaceous, it is generally conformable on the Pennsylvanian basal limestone but in some cases unconformable on Ellenburger highs, as in southwest Bandera County.
The isopach contours, together with isoliths and lithologic percentage maps of the sands, shales, and local carbonates, focus on a southern source for sediments and document at least two sharp movements of the Ouachita Thrust in a northward direction. Sands approach 50% of the section along the north face of the thrust. Numerous shows of gas are present, and production has been established in Kimble, Sutton, and Edwards counties. These clastics serve as source, seal, and reservoir.
The Cretaceous section is 1,500-2,000 ft thick and made up of limestone plus shale and sandstone that onlap generally flat-lying older rocks. It has proven an acceptable reservoir with production found in northwestern Edwards County.
CONCLUSIONS
Regional studies display both positive and negative attributes of large areas and enable the explorationist to focus his attention on the most worthwhile objectives at the best locations. Results of this review may be summarized as follows:
The basin is a dominantly oil province, although shows also suggest gas potential.
Potential reservoirs include the entire section from Cambrian upward through the Ordovician Ellenburger limestone, a Pennsylvanian basal limestone section, and a Permo-Pennsylvanian clastic sequence.
Relatively shallow drilling, generally to less than 7,500 ft, is needed to test the section into the top of Cambrian.
Minor production of oil and gas and good oil and gas shows confirm the petroliferous nature of the rock sequence. This in turn indicates sufficient maturation of the rock column to produce oil and gas, probably in commercial quantities.
A review of previous wells indicates short tests and inadequate treatment of zones with excellent shows.
The stratigraphic column exhibits source, seal and reservoir combinations.
Porosity development has been enhanced by sub-aerial erosion, dolomitization, solution, and fracturing. Fracturing probably offers the best opportunity for production.
A variety of traps increases the area's exploration potential. Erosional highs, anticlines, horst blocks, bald-headed structures, wedgeouts, fault traps, fracture zones, and combinations of all of these are possible.
Minor production, near misses, and numerous shows in sediments of the Kerr basin suggest that the poorly explored area has an economic potential yet to be realized. The area has numerous problems that make exploration difficult, but like most areas in the U.S. an increase in the price per barrel of oil will make exploration here more attractive.
Kerr and surrounding counties have not been adequately explored, and the potential for production should be of interest to independents and companies alike.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.
