Carbonate reef prospect identification in a mature field: East Vealmoor Field, Horseshoe Atoll Reef complex, West Texas, USA David Balogh, Doug Klepacki, Prism Seismic, Centennial CO, USA

“Wells 7 and 17 contain the highest permeability reef in the field; the Prism impedance and spectral imaging work form a solid match with these wells and a good match with other production in the field." Operator – East Vealmoor Field

Introduction Prism seismic applied high-resolution inversion to a 3D seismic volume from East Vealmoor Field. The field produces from the Canyon Reef interval on part of the larger Horseshoe Atoll reef complex in the Midland Basin of West Texas. Carbonate reservoirs in this basin have been producing since 1938, however, the historic market value of oil has limited the amount of resources applied to locating and extracting the remaining accumulations, especially in small fields where production has declined to stripper-well levels. With current oil prices, however, 3D seismic for detailed reservoir studies can be economically employed to find remaining reserves in these mature fields. In this case history, new locations for in-fill drilling within a portion of East Vealmoor Field, where many wells had been abandoned, were defined by Prism’s high-resolution impedance inversion integrated with well and geologic control. High-Resolution Inversion Technique The objective of seismic inversion is to convert band-limited, wiggle-trace seismic data into acoustic property (impedance) traces. In this form, impedance may be related to other petrophysical properties of greater interest to reservoir modeling such as porosity and lithology. The seismic response to inter-well heterogeneities is transformed by inversion to variations in these properties that are critical to reservoir characterization. Model-based impedance inversion is used by Prism to evaluate the porosity of the Canyon Reef carbonate buildup. A well-seismic tie process is performed between synthetic traces at the wells and the 3D seismic volume. Impedance from the log data is interpolated throughout the 3D area to populate the a priori model. Inputs to the high-resolution inversion include the a priori impedance model, 27 Hz 0 degree phase Ricker wavelet, 3D seismic volume re-sampled to 0.5 ms, and time horizons for the interpreted seismic reflections near the Canyon Reef interval. East Vealmoor Field, Horseshoe Atoll Complex, Midland Basin, Texas

Production history in East Vealmoor indicates that the typical successful well produces 200,000 - 500,000 bbls of oil over the 30-50 year life of the well. Within the bounds of this study almost all of the wells containing Canyon Reef reservoir contain sustained production, however, cumulative production per well varies from just a few thousand to over 600,000 bbls. Historically, prospecting has been for structurally high reef having high porosity with good communication throughout the reservoir. However, in the study area the porosity is not evenly distributed, so separate producing compartments may exist anywhere on structure resulting in highly variable cumulative production between wells. Production from East Vealmoor has reached a mature stage but future prospects are to be found as “attic oil” either in local structural highs with high porosity or in isolated pockets of porosity without structural advantage. This makes imaging porosity the primary aim of Prism’s work. Prism’s prospect analysis focused in three areas:

1) Structural: Use time horizons and depth surfaces to identify present-day structures

2) Topographic: Use isochroning to identify onlap/thinning over reefal buildup related to paleo-topography

3) Lithological: Use the high-resolution impedance inversion to find zones of higher porosity

Structural prospects should reside above current production levels to tap “attic” oil. Several “attic” structures are identified in Figure 1, which shows the top of the Canyon Reef structure as subsea depths. Most notable is the 25 foot “attic” structure, Prospect-A, identified between wells 12 and 13, which is analogous to the highly-productive structure near wells 6, 7, and 17.

Figure 1: Canyon Reef depth structure. Contour interval is 5 feet. Prospects are structural and contain "attic" reservoir above current production. Reef build-up may be identified both by thickening of the Canyon Reef interval due to reef growth and by thinning of beds above the reef buildup. (See Figure 2)

10ft “attic”

25ft “attic” Prospect A

Figure 2: Thickened buildup of the Canyon Reef combined with post-reef thinning highlights reef structures. The seismic interpretation includes “above_REEF” and “below_REEF” horizons used to construct isochrons above and below the top of the “Canyon Reef” horizon. The ratio of the lower isochron to the upper isochron will emphasize zones of reef build-up. This paleo-topography attribute shown in Figure 3 highlights areas of elevated probability of reef buildup; warm colors correspond to increasing probability.

Figure 3: Ratio of temporal thickening below the reservoir to temporal thinning above the reservoir. The Topography attribute will emphasize zones of paleo-structural highs with post-depositional thinning. Canyon Reef depth structure contours are overlain with a contour interval of 5 feet.

Prospect A

High porosity is known to correlate with low bulk density and low velocity. Impedance is the product of density and velocity, so low impedance in the reservoir interval could be indicative of elevated porosity. This relationship between low impedance and high porosity is empirically confirmed by correlation of impedances from Prism’s inversion at wells where porosity is estimated from limited log control and cumulative production. Mean impedance in the upper 10ms of the Canyon Reef seismic interval is shown in Figure 4.

Figure 4: Mean impedance in the upper 10ms of the Canyon Reef. Low impedance (warm colors) should correspond to high porosity in the wells. Canyon Reef depth structure contours are overlain with a contour interval of 5 feet. Although historical production has drawn the original oil/water contact to essentially the highest producing level in those wells, it has been demonstrated, according to the field operator: “In a five spot pattern between wells the coning effect and drainage has been demonstrated by open hole logs and production to lower the oil water contact by 40 feet”. Significant oil column can still exist especially in the “attic” structurally high to the surrounding wells. These are depicted as the green to dark-green zones in Figure 5.

Figure 5: Depth-structural map showing remaining “attic” reservoir accumulation – Top of Canyon Reef. Blue/gray contact is the approximate original oil/water contact. Overlain in green to black filled contours are residual structural highs in which “attic” oil is likely to exist. Conclusions In general, zones of elevated structure above historical production are prospective in East Vealmoor field. Analysis of paleo-topography may discriminate between true reef build-up and apparent structure with less reservoir potential. Structural prospects for infill drilling can then be high-graded by Prism’s inversion using impedance to depict elevated porosity. Applying these criteria, a possible reef build-up with “attic” accumulation is dramatically demonstrated in Figure 6 for Prospect A. For reference the “Canyon Reef” time horizon shifted 20 ms is shown along with the vertical well bores of surrounding wells. The proposed infill well for Prospect-A is shown with a dashed wellbore.

Figure 6: 3D rendering of low-impedance zone within the seismic volume. The proposed well, located in “attic” structure with lowered impedance and favorable paleo-topography, is interpreted to be high-porosity reef buildup. Prism’s prospecting workflow developed for 3D seismic in Midland Basin, Horseshoe Atoll Canyon Reef includes structural interpretation, topographical considerations, and lithological analysis. In East Vealmoor field, high-porosity “attic” reservoir is identified at Prospect-A thus adding significant value to a mature asset. The use of high-resolution inversion in advanced field studies has become applicable to smaller, mature fields given the current economic incentives of high oil prices and the potential of adding significant reserves.

Prospect A