Robert T. Ryder
U.S. Geological Survey
Reston, Va.
Myung W. Lee
Warren F. Agena
U.S. Geological Survey
Denver
Robert C. Anderson
Berrong Enterprises Ltd.
Golden, Colo.
The east flank of the Rock Springs uplift and the adjacent Wamsutter arch contain several large hydrocarbon accumulations.
Among these accumulations are Patrick Draw field, which produces oil and gas from a stratigraphic trap in the Upper Cretaceous Almond formation 1 2 3 and Table Rock field, a faulted anticlinal trap that produces gas from multiple Tertiary, Mesozoic, and Paleozoic reservoirs 4 5 (Fig. 1).
The principal petroleum reservoir in Patrick Draw field is a sandstone at the top of the Almond formation. This sandstone attains a maximum thickness of 35 ft (11m) and pinches out westward into relatively impervious siltstone and shale that constitute the trapping facies.
The objective of this investigation is to determine whether or not the stratigraphic trap at Patrick Draw can be detected on a 12 fold, common depth point seismic profile acquired by Forest Oil Corp. and its partners. The seismic line is 18.5 miles (29.6 km) long and crosses Patrick Draw and Table Rock fields.
Detailed geologic maps,6 7 8 numerous oil and gas exploration holes drilled into the Upper Cretaceous sequence, and several oil and gas exploration drill holes as deep as 19,466 ft (5.9 km) drilled into lower Paleozoic and Precambrian rocks in the Table Rock area allow specific stratigraphic horizons and (or) sequences to be accurately tied to the seismic record.
Seismic and geologic data used in this investigation supplement the regional seismic profile across the Rock Springs uplift published by Stearns et al.1 and Garing and Tainter10 and the detailed subsurface stratigraphic studies of the Upper Cretaceous (part) and lower Tertiary sequences published by Weimer1 and McCubbin and Brady.2
ACQUISITION AND PROCESSING
The seismic line used in the investigation is the arch area line No. 5 collected in May 1973 by Geophysical Services Inc. for Forest and partners.
An array of four vibrators provided the seismic source at a sweep frequency of 1448 hz for 14 sec; the resultant seismic energy was recorded for 19 sec at 4 ms intervals.
The vibrator array occupied every other recording station and was spaced 330 ft (100.5 m) apart. This recording geometry provided a maximum fold coverage of 12.
Line No. 5 trends approximately S 85 E across the east flank of the Rock Springs uplift and traverses, from west to east, gently east-dipping strata of the Upper Cretaceous Lewis shale, Upper Cretaceous Fox Hills sandstone, Upper Cretaceous Lance formation, Paleocene Fort Union formation, and the Paleocene and Eocene Wasatch formation (Fig. 2).
Geologic structure here is relatively simple and consists of homoclinal strata, which dip 1-6 eastward, and of several N 70-80 E trending transverse faults having a normal vertical separation between beds as much as 330 ft (100 m) in the western part of the study area and decreasing to 33 ft (10 m) or less in the vicinity of Patrick Draw field .1 6 7 11
In 1977, Forest donated line No. 5 to the U.S. Geological Survey for seismic-stratigraphic investigations of Patrick Draw oil field. Magnetic data tapes, a shotpoint (SP) location map, and an unmigrated section processed by GSI were included in the transaction.
The profile was reprocessed in 1978 and 1986 by the USGS. Important steps in the processing sequence included single window trace equalization, spike deconvolution with a 160 ms operator length, application of a 10-48 hz filter, datum and surface consistent residual static correction, migration, and the phase correction using the Hilbert transform. The seismic profile was plotted from a flat datum of 6,500 ft above sea level.
IDENTIFICATION OF SEISMIC REFLECTIONS
The Texaco Inc. 15 Table Rock Unit gas well, Well 21, and accompanying borehole logs, which sampled various rock properties from the Paleocene and Eocene Wasatch formation near the surface to the middle Pennsylvanian Tensleep sandstone at a total depth of 17,337 ft (5.3 km), provided velocity and stratigraphic data for identifying major seismic reflection horizons on line No. 5.
The synthetic vertical seismic profile and the accompanying synthetic seismograms (Fig. 3), used to tie strata in the Texaco 15 well with reflections on seismic line No. 5, were calculated and plotted from sonic log data recorded between 1,006 ft and 16,337 ft (0.3 and 5.3 km).
The authors chose the synthetic vertical seismic profile 12 rather than the conventional synthetic seismogram13 " for the seismic-to-stratigraphy tie because the vertical seismic profile format treats depth as a linear function, whereas the conventional synthetic seismogram format treats depth as a nonlinear function.
By preserving depth as a linear function, acoustic and lithologic log data can be plotted in an undistorted form.
Furthermore, the synthetic vertical seismic profile records the evolution of the seismic wave field so that seismic reflections recorded at the earth's surface can be traced downward to their point of origin and can be correlated with specific stratigraphic horizons identified on borehole logS.15 16
The synthetic vertical seismic profile and accompanying synthetic seismograms for the Texaco 15 well involved the following procedure:
- Interval transit time values on the sonic log were digitized, sampled at 2 ft (0.6 m) intervals, and displayed on a cross plot of velocity and depth.
- Abnormally high and low velocity excursions due to factors such as borehole irregularities, cycle skipping, and intermittent sticking of the logging tool to the borehole wall were edited statistically and removed.
- Acoustic impedance functions were calculated for the strata in the borehole by using the digitized velocity data. No density data were recorded in the well. Density was assigned a constant value of 1.0 g/cc in the calculations.
- By using methods developed by Lee, 17 the theoretical total wave field (upward and downward traveling components) recorded at 50 ft (15 m) intervals in the well were calculated and band-pass filtered at frequency values ranging from 8/12 to 32/48 hz to 8/12 to 68/96 hz.
- The resultant synthetic vertical seismic profile was plotted using depth as the vertical axis and time as the horizontal axis. Synthetic reflected seismic wave fields recorded in the borehole at or near the earth's surface for four specific bandwidths (8/12-23/48 hz, 8/12-42/56 hz, 8/12-50/68 hz, 8/12-68/96 hz) were plotted in a horizontal position at the top of the profile. These wave fields correspond to conventional synthetic seismograms. 13 14 Normal polarity, where a peak waveform (excursion to viewer's right) results from a unit of low acoustic impedance overlying a unit of higher acoustic impedance, is maintained in all the plots.
CORRELATIONS
Major reflections on the synthetic seismograms were correlated with known stratigraphic units by tracing them downward, along the path of the upward-traveling reflections to their depth of origin.
A generalized American Stratigraphic Co. lithologic log was plotted alongside the velocity log to aid in the identification of stratigraphic units. Stratigraphic nomenclature used by Ryder18 is used in this investigation.
Many of the high amplitude reflected events on the synthetic vertical seismic profile correspond to the tops of formations such as the Fox Hills sandstone, Almond formation, Baxter shale, Cloverly formation, Sundance formation, and Phosphoria and Park City formations.
Other high amplitude reflections correspond to units within formations, such as coal zones in the Fort Union and Rock Springs formations and unnamed units or sequences in the Lewis and Baxter shales. The series of high amplitude reflections on the synthetic vertical seismic profile resulting from a 9001,000 ft thick (275-305 m) coalbed sequence in the lower part of the Fort Union formation resembles events in the Piceance basin of Colorado resulting from two 25-40 ft thick (7.5-12 m) coalbed sequences in the Upper Cretaceous Mesaverde group.19
The synthetic seismogram derived from the Texaco 15 well was projected northeastward about 3.3 miles (5.3 km) along the crest of the Table Rock anticline into seismic line No. 5 at approximately SP 270 (Figs. 3, 4). Synthetic seismogram "a," band-pass filtered at 8/12 to 32/48 hz, was used because it most closely approximates the frequency content of the seismic profile.
All the reflections identified by stratigraphic names on the synthetic seismogram can be matched with reflections in line No. 5 either at the projected well site or 1-5 miles (1.6-8 km) westward where data quality improves owing to increased fold coverage (Fig. 4). Moreover, in most cases, the relative amplitudes of the reflections on the synthetic seismogram match those on line 5.
Time shifts of 5-20 ms are required for the best fit between the theoretically and field-derived wave forms. The synthetic seismogram was aligned with the field seismic data so that a zero time shift (one-to-one match) occurred at the unnamed coal zone in the Rock Springs formation (Fig. 4). Above and below this horizon, progressively greater stretching of the synthetic seismogram was required for a one-to-one match with the field seismic data.
STRETCHING REQUIRED
Approximately 20 ms of stretching was required to match the Phosphoria and Fort Union horizons on the synthetic seismogram with respective horizons on the field seismic data, for a total of 35-40 ms of stretching of the entire Fort Union to Phosphoria interval.
These required time shifts are the result of sonic log velocity values for a given stratigraphic interval being slightly greater than the seismic velocity values. Stratigraphic variation in the 3.3 mile (5.3 km) distance between the Texaco 15 well and the seismic profile probably is not a contributing factor to the observed velocity differences.
Major reflection horizons on seismic line No. 5 from the middle part of the Baxter shale and above in the vicinity of the projected Texaco 15 well are traced easily westward as far as SP 75, beyond which they largely fade into a zone of poor quality data (Fig. 4). In contrast, major reflections from below the middle part of the Baxter shale are traceable across most of the profile but tend to lose their continuity in the vicinity of the Table Rock anticline.
Although the Mississippian Madison limestone and Precambrian basement rocks were not reached in the Texaco 15 well, their reflections are interpreted on line No. 5 on the basis of velocity and thickness data in the nearby Texaco 23 well (Fig. 2, well 22) drilled 19,446 ft (5.9 km) into Precambrian basement rocks .5
The authors' pick of the top of the Madison limestone on line No. 5 is supported by its 400 ms two-way travel time (TWT) separation with the overlying top of the Upper Jurassic Morrison formation, a value that is comparable to the TWT of the Morrison to Madison interval on the nearby regional seismic profile interpreted by Stearns et al.9 and Garing and Tainter.10
The top of the Precambrian basement rocks on line No. 5, picked at the lowest continuous reflection horizon about 270-300 ms below the proposed Madison reflection, is consistent with velocity and thickness data recorded in the Texaco 23 well.
Next: Detection of the stratigraphic trap.
Copyright 1990 Oil & Gas Journal. All Rights Reserved.