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First Choice Prospect
Aiken Prospect
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The First Choice Prospect, Tom Green County, Texas


OVERVIEW:

 

The First Choice Prospect is located three miles west of the town site of Wall, and six miles southeast of the City of San Angelo, Texas just east of the Smith and Caraway field. The prospect is a closed anticlinal feature at Strawn limestone level developed on a faulted pre-Pennsylvanian uplift as shown by seismic and subsurface data. Deeply eroded Cambro-Ordovician beds are at its core, and the Lower Pennsylvanian may be in contact with the pre-Cambrian at its center. A coincident structure at Middle Permian Clear Fork level has been mapped by stratigraphic test drilling. This is one of a series of faulted structures extending southward from the Harriett Field area east of San Angelo. These structures produce from multiple Pennsylvanian and Ordovician intervals.


HISTORY:

 

Prior to 1940 this area was relatively unexplored. In the late 1940s Plymouth Oil Company explored several areas in Tom Green County where there was open acreage and they could obtain permits and options for leases at low cost. The exploration method was shallow stratigraphic test drilling ("core drilling") to map anomalous shallow structure, followed by several lines of single fold seismic to confirm coincident deep structure. This method led to the discovery of the large Susan Peak (Strawn limestone) oil field by Plymouth and Benedum-Trees Oil Companies in 1948.

In 1955 Benedum-Trees drilled their Debus on a incompletely mapped lead 3 miles North as they had it mapped. The well was a disappointing Ellenburger dry hole. Of interest, though, was the low structural position of the well and the 300 feet of anomalous Atoka section that was penetrated between the Strawn and Ellenburger. We now know that this thick Atoka is present in the narrow grabens that form the east border of many of the structural traps along the Eastern Shelf of the Midland Basin including this prospect.

In 1950 Shell Oil, using a contraflush drilling rig to drill and map the shallow beds followed by a single fold seismic program, discovered the Harriett (Canyon and Strawn Limestone) field. To the south along the trend the Pecan Station (Canyon limestone) field was discovered in 1953 by C. L. McMahon on a Stanolind farmout. Stanolind had acquired leases on a shallow anomaly mapped by core drill. A consulting geologist later identified a surface drainage anomaly coincident with the shallow structure. The same year the Floyd Miller (Cisco limestone) field was discovered by Tucker Drilling on a farmout from Shell that was based upon Shell's stratigraphic test information. These discoveries and the success of other companies core drilling on their own leases led Shell and several other majors to begin extensive stratigraphic drilling programs along the entire length of the Eastern Shelf.
This renewed mapping revealed a series of Middle Permian structures extending from the Harriett field, seven miles north of FIRST CHOICE , to the Pecan Station field six miles south of FIRST CHOICE . A limited amount of single fold seismic work by Shell confirmed this trend. A prominent culmination along this trend was mapped just west of the Benedum-Trees, Debus well, but no leases were taken as the acreage was held by Benedum-Trees et al. Shortly thereafter Shell reduced it exploration effort on the Eastern Shelf and this structure has never been drilled.

Detailed mapping of the Middle Permian incorporating the stratigraphic test data with data from the numerous wells drilled along the Eastern Shelf has emphasized the significance of these structural trends. Fortune Production Company drilled a prospect formed by a culmination along the trend south from Harriett, and in 1987 they completed the discovery well of the Harriett South field. Fortune then drilled another prospect on a parallel trend to the east and discovered the Schneirs (Strawn limestone) field. Also along this trend are now located the Veribest (Strawn limestone), and Veribest South (Ellenburger) fields.


GEOLOGY:

 

Pre-Pennsylvanian erosion along this part of the Eastern Shelf stripped off the sediments down to the Lower part of the Ordovician Ellenburger. On some structures the Ordovician and Cambrian sediments were entirely removed, leaving a pre-Cambrian core in direct contact with the overlying Pennsylvanian. This is the case at the Pecan Station field structure, and it may be the case at FIRST CHOICE.

Renewed structural activity in earliest Pennsylvanian time created a series of low relief structures and a north trending normal fault system extending from Sutton County through Stonewall County along the entire central and eastern portion of the Eastern Shelf. The dominant trend of faulting and associated structures has become known as the San Angelo Axis, or the Fort Chadbourne Trend. Narrow grabens characterize some of this faulting, and as the regional dip is now to the west these grabens form a key part of the trapping mechanisms on the east side of fields producing from the Pennsylvanian and Ordovician age rocks.

Detritus from the eroding Ellenburger filled the grabens. Renewed movement during earliest Pennsylvanian time allowed Atoka sediments to be preserved in the upper part of these grabens below the thick limestones of the Strawn. Encountering a thick sequence of Atoka and detrital beds along the Eastern Shelf is an indication that the well may be in one of these grabens flanking a structure.

The Benedum-Trees well 3 miles north and on the east side of FIRST CHOICE ran structurally low on all markers and penetrated a thick Atoka and detrital fill section, indicating its position is in one of the grabens. Another graben exists in the structurally complex area west of FIRST CHOICE. The Hickey, Stanford well northwest of FIRST CHOICE penetrated 700 feet of Atoka and detrital fill sediments above the Ellenburger.

The Benedum-Trees well 3 miles north and on the east side of FIRST CHOICE ran structurally low on all markers and penetrated a thick Atoka and detrital fill section, indicating its position is in one of the grabens. Another graben exists in the structurally complex area west of FIRST CHOICE. The Hickey, Stanford well northwest of FIRST CHOICE penetrated 700 feet of Atoka and detrital fill sediments above the Ellenburger.

Both well and multiple stratigraphic tests controls define a Middle Permian structure. A coincident structure is mapped at Strawn limestone level using well control and flanking seismic data. These structures are on trend with the producing Harriett and Harriett South structures, the untested South Harriett prospect, the untested Simecek and Jett Prospects, and the proven Pecan Station field structure.


GEOPHYSICS:

 

American Trading and Production Company (ATAPCO) conducted several hundred miles of reconnaissance and detailed CDP seismic work in Tom Green County during the early 1980s. One of these programs involved a block of leases north of FIRST CHOICE, and several of these lines extend to the flanks of the FIRST CHOICE Prospect. One line, Line 5, is of particular interest as it passes along the west side of Simecek and the center east side of FIRST CHOICE and shows both the bordering fault and the probability that FIRST CHOICE has pre-Cambrian below the Pennsylvanian in the center of the structure similar to the Pecan Station structure.

Additional seismic lines shot by ATAPCO across the FIRST CHOICE Prospect in 1981. These were 24 fold CDP lines generally fair quality data. Line 1 runs north to south and parallels line 5 and bounds the central west side of the prospect. Line 7, 3 and 4 run west to east and tie in line 1 and 5 across the FIRST CHOICE Prospect. Line 3 runs through West #1 Wilde well in section 53. Line 7 runs through both Stanolind 1A-Green in section 53 and the Palmer #1 Oates in section 52.

Additional tools of RAD data Reeves Exploration and radiometric using the Ludlum Measurements Inc. "Model 4404-16 Geo-Explorer" were used to show the presence of hydrocarbon seeps possible in the structures with success.


STRUCTURE:

 

The FIRST CHOICE Prospect is a Pennsylvanian and pre-Pennsylvanian feature that is on trend with and part of a series of such features that extend from Fisher County on the north to Schleicher County on the south. Many of these features produce oil and gas from multiple pays. At West, a northeast trending horst some 6,000 feet wide extends to connect with the main north trending fault and graben system just east of the prospect area. Displacements along the bounding pre-Pennsylvanian faults at West are 100 to 150 feet. A closed structure at Pennsylvanian Strawn limestone level is developed on the southwestern portion of the horst. A loss of Strawn limestone signal on several seismic lines over the crest of the structure suggests the possible development of porous reefing. The closure at Strawn limestone level has some 150 feet of relief and covers some 825 acres.


OBJECTIVES:

 

The primary objectives at First Choice are the Strawn and Canyon limestones that produce at Susan Peak, Pecan Station, Harriett, and Harriett South fields. Secondary objectives include the Cisco limestone that produces in the Floyd Miller field, the Canyon and Strawn sandstones, and the Ellenburger dolomite; all of which produce in this area. Of speculative interest is the weathered pre-Cambrian granite that may be at the core of the structure. This formation yielded 1,000,000 barrels of oil from one well in the White Flat field of Nolan County where it was encountered below the Pennsylvanian.

Great Western, Washington CSL, on the southwestern flank of the Strawn limestone structure at FIRST CHOICE, cored fractured limestone with oil stain and fluorescence in the Canyon Palo Pinto limestone, and recovered 2,800 feet of gas in the drillpipe. On a drillstem test of the Strawn limestone the recovered 140 feet of oil and gas-cut mud, and 210 feet of gas cut saltwater. An earlier test of the Strawn limestone has recovered 4,230 feet of salt water. These tests show the Strawn limestone at FIRST CHOICE to be both porous and hydrocarbon bearing.

Per well recoveries from the primary pays at FIRST CHOICE are estimated to average of 100,000 barrels of oil per 40 acre location. The estimated total recovery from all pays, assuming a field size of 880 acres, is 2,500,000 barrels. The depth to test all objectives is 6,200 feet. The main Strawn limestone pay will be encountered at about 5,200 feet.


LAND:

 

Principal acreage of 485.658 Acres with 8 to 12 locations potential is under lease for operations with additional acreage available as needed for field development.


RECOMMENDATION:

 

A 6,200 foot Ellenburger wildcat should be drilled inside the NE corner of section 7. The first 2 to 4 well locations are set on raw undeveloped area with mesquite and cedar brush land with no timing issues for drilling operations. Additional locations cover two thirds of the surface on level farm land under cultivation for cotton crops. Although access is easy from nearby paved highways, surface operations must consider the planting and harvesting times and the possibility of above ground and in-ground irrigation equipment.