Aeromagnetic Survey

Integrated Geologic Interpretation

Wykert Sand Trend in the D-J Basin

INTRODUCTION: EDCON-PRJ is marketing a localized aeromagnetic interpretation of the Wykert Sand Trend in the Denver-Julesburg Basin. The aeromagnetic analysis has been integrated with subsurface data and mapping provided by Consulting Geologist John C. Webb.

The study area covers 4,360 line-miles or approximately 40 townships, half of which are located in Kimball County, Nebraska. (See map on cover.) Included in the study boundaries are several northern townships in Banner, Morrill and Cheyenne counties, Nebraska, and nine townships to the south in Weld and Logan counties, Colorado.

The purpose of this integrated interpretation is to identify basement structure and related fracture enhancement that have influenced reservoir potential and deposition of the Wykert Sand and underlying Admire Group (Permian/Wolfcamp age) in the Wykert Sand Trend. The Wykert Sand has been interpreted by Ackman, et al. (1993) as an accumulation of sand along a northward trending shoreline which existed to the east of the structurally-influenced Lusk Embayment. EDCON-PRJ and John C. Webb believe that, in addition to regional structural influence on the Wykert Sand Trend, local structural features rooted in the Precambrian basement have significantly influenced deposition and have localized a diagenetic overprint that is responsible for preservation of porosity and permeability in the Wykert Sand. The underlying Admire Group also appears to have been strongly influenced by Precambrian structures. Therefore, this study has focused on the influence of paleostructures on patterns of thickness, porosity development and trapping mechanisms that are responsible for production from the Wykert Sand and Admire Group.

Two of the more controversial issues pertaining to a paleostructural analysis of this area regard whether aeromagnetics can locate a relatively thin sand body at a depth of approximately 8,000 feet, and whether paleostructure can influence deposition and porosity development of the Wykert Sand and adjacent carbonates. The first issue regards whether aeromagnetic techniques of interpretation can accurately locate basement structure. Aeromagnetic data have been successfully applied to the identification of structure in the Precambrian basement of the Nebraska Panhandle and other areas. Of interest is Texaco's discovery of Swearingen Field which was drilled on an aeromagnetic anomaly detected by the 1987 aeromagnetic survey conducted by EDCON-PRJ, LSSI and Airmag Surveys.

Weimer (1978 and 1980) addressed the second issue in an article showing the relationship between Cretaceous sand trends and the paleotopographic highs and Precambrian basement structure. Sonnenberg and Weimer (1981) demonstrated paleotopographic and structural control on lithofacies and production for the Lyons sandstone (Permian, Leonard age). Numerous authors (Garfield, et al., 1988; Hoyt, 1963; Levandowski, et al, 1973; and Momper, 1963) have documented the effect of paleostructural elements, including the Transcontinental Arch, on lithofacies and reservoir potential in the Paleozoics of the northern D-J Basin.

As noted earlier, EDCON-PRJ is familiar with this area as key personnel conducted a larger regional analysis in the Nebraska Panhandle in 1987. EDCON-PRJ has used high-sensitivity, closely-spaced (½ mile by mile grid) aeromagnetic data to map basement structure and faults which influenced the paleotopography during the deposition of the Wykert Sand, Admire Group and the adjacent carbonates.

Due to the presence of these paleotopographic highs and lows, and the ability of properly processed and interpreted aeromagnetic data to target these features, we believe that EDCON-PRJ's detailed aeromagnetic survey coupled with the geological subsurface data will be more beneficial to explorationists here than a broad grid of seismic profiles is likely to be. To facilitate this undertaking, EDCON-PRJ has used its newly-developed neural network technique (recently patented by Texaco) which has high resolution basement structure mapping capabilities that help quantify subtle relief to a degree not previously possible. The reason PRJ can quantify basement structure more accurately is found in the power of the neural network pattern-recognition capability to recognize subtle structural anomalies even in the presence of intrabasement susceptibility variations. The neural-network technique is far superior to pre-existing computer and "eyeball" anomaly recognition techniques.

Log data from four multi-well fields, Kleinholz, Terrestrial, Anna and Swearingen, have been included in our study. Kleinholz, originally opened in 1986, has been the primary focus of recent drilling activity. Cumulative production, through April 1993, from its 28 wells was 1.68 million barrels of oil and 578 MMcf of gas from the Wykert Sand and Admire A, B and C zones primarily between 8,000 and 9,000 ft. Exxon Co. USA, Advantage Resources and Bruce C. Evertson are field operators.

The nine-well Terrestrial Field, opened by Exxon in 1990, has produced approximately 181,000 bbls of oil and 51 MMcf of gas from the Wolfcamp and Wykert sands. Exxon, New London Oil and Western Operating are field operators. Anna Field has contributed three producing wells to the economics of this play. Cumulative production is nearly 137,000 bbls of oil and 10,340 Mcf of gas. The fourth field, Swearingen, has cumulative production of nearly 73,000 bbls of oil and 107 Mcf of gas from two wells that have produced for nearly five years and one year, respectively.

Two recent discoveries, the first wells to delineate Kreitzinger and Long fields, are also included in the study. These wells which produce from Wykert and Admire, respectively, are located approximately 8 (Kreitzinger) and 12 miles (Long) southeast of Kleinholz Field.

Log tops from a total of 149 wells in this area were incorporated in the study to aid in mapping. Regional cross-sections were constructed running east-west and north-south (parallel and perpendicular to trend) through Kleinholz, Terrestrial, Mamoo, Swearingen, Anna, Long and Kreitzinger fields. See the list of deliverables below for more details regarding the scope of the study.

USEFULNESS OF AEROMAGNETIC DATA

Aeromagnetic data are very useful for determining basement structure because Precambrian basement has a measurable magnetic structure whereas the overlying section is almost totally nonmagnetic. Thus, magnetic surveying can "see" through the nonmagnetic sediments. Detection of faulted basement structures along aeromagnetic profiles requires the separation of low-amplitude and high-frequency suprabasement (structural) anomalies from high amplitude and low-frequency intrabasement (compositional) anomalies. These data have defined regional trends of present day basement structure and have been integrated with isopach and reservoir trend maps to help EDCON-PRJ determine the effect of basement structure on hydrocarbon migration and entrapment.

This integrated aeromagnetic study includes the following key components:

1. The incorporation of approximately 4,360 line miles of EDCON-PRJ-processed high-resolution, low-altitude aeromagnetic data with a one-half by one-mile line spacing grid.

2. Up to 20 formation tops were picked per well between the top of Paleozoic and basement from all available well log information. Deep wells that do not penetrate Precambrian basement were mapped on a deep Pennsylvanian marker and projected to Precambrian basement. These tops were integrated into mapping and are available as a separate database from John C. Webb.

3. An integrated interpretation of the aeromagnetic data, well control and existing Paleozoic production was compiled. Cross-section models over Kleinholz and Terrestrial oil fields quantify the magnetic signatures of structural features and lateral composition changes within basement. Total Paleozoic, Admire Group and Wykert Sand isopach maps, with the latter two showing net sand porosity, are included. The contoured Precambrian basement structure map delineates structural features, faults, lineaments and petroleum prospects.

AEROMAGNETIC SURVEY

Most of this high-resolution survey was flown in 1987 by Airmag Surveys, Philadelphia. Airmag flew the southern (Weld County, Colorado), western (Kimball County, Nebraska) and northeastern (Morrill County, Nebraska) legs in May of 1993. Flying was at a constant terrain clearance of 300 ft. Using a sampling rate of 0.25 seconds, the earth's magnetic field was sampled at an approximate ground spacing of 60 ft. Positioning of the flight paths was performed using photographic strip charts correlated to 7½-minute topographic quads. A continuously recording base station magnetometer provided diurnal field variations for identification of visibly-undetectable magnetic storms and for removal of diurnal field variation.

Data acquisition specifications were as follows:

Airborne magnetometer: Optically-pumped Cesium Vapor Magnetometer with 0.01 gamma sensitivity

Ground magnetometer: Schoenstedt. Portions of lines acquired while the base station magnetometer exceeded a three gamma deviation on the three minute cords were reflown. Data acquisition was not attempted during periods of K activity of 4 or greater.

Sample Rate: 0.25 second resulting in approximately 60-ft samples

Positioning: Photographic strip chart correlated to 7½-minute topographic quads for 1987 acquisition and G.P.S. navigation for 1993 acquisition

Survey Altitude: 300 ft above ground

Radar Altimeter: Honeywell 7505

Camera: Flight research strip

Recorders: Two H.P. 7130

Aircraft: Cessna 320 with stinger

DATA PROCESSING

EDCON-PRJ employed state-of-the-art aeromagnetic data processing procedures to retain the high sensitivity information. The following steps were performed.

• Flight line recovery data were checked for heading and velocity errors.
• Data tares and spikes were removed.
• Flight line and tie line intersections were minimized by a least-squares polynomial line adjustment method.
• IGRF correction was performed using the WC85 and WC90 formulas extrapolated to the acquisition date

DELIVERABLES

The following list of maps (at a 1" = 8,000-ft scale), illustrations and data sheets will be provided.

Data Products

• Flight line location map (mylar)
• Digital data tapes
• Processing report

Interpretation Products Provided By EDCON-PRJ (using neural network technology)

• Total Magnetic Intensity map (paper)
• Two shaded-relief images (representing different sun angles) (laminated paper)
• Regional depth-to-Precambrian basement structure contour map with faults and graded prospective areas using the STARMAG^TM neural network system (mylar)
• Lineament pick overlay incorporating boundary picks, faults and structural axes from the three dimensional work station display and shaded relief images (mylar)

Geologic Mapping Provided By Consultant John C. Webb

• Total Paleozoic Isopach map (b/w on paper)
• Admire Group Isopach and Net Porosity map (b/w on paper)
• Wykert Sand Isopach and Net Porosity map (b/w on paper)
• Regional Cross Sections running parallel and perpendicular to trend (paper)

In addition, a report summarizing the significant findings of this study was compiled jointly by EDCON-PRJ and Mr. Webb. This report contains a Prospective Areas chart which has graded areas (such as, good, fair or poor), interpreted structural relief and estimated net porosity/isopach thickness.