Geological and Petrophysical characterization of the Ferron Sandstone for 3-D simulation of a fluvial-deltaic reservoir

OBJECTIVE

SUMMARY OF TECHNICAL PROGRESS

The primary objective of the Case Studies Task is to develop a detailed geological and petrophysical characterization, at well-sweep scale or smaller, of the primary reservoir lithofacies typically found in a fluvial-dominated deltaic reservoir. Interpretations of lithofacies, bounding surfaces, and other geologic information are being combined with permeability measurements from closely spaced traverses and from drill-hole cores to develop a 3-D view of the reservoirs within three case-study areas (locations shown on figure 1).

Regional Stratigraphy

Collection and Interpretation of Existing Surface and Subsurface Data The UGS has completed collecting and compiling available published and unpublished maps, measured sections, well logs, core descriptions, reports, and other data. There are 486 wells in the study area, of which 413 were cored. By December 31, 1994, the UGS had acquired 138 geophysical logs and 1,800 feet (550 m) of core or core descriptions from these wells (table 1). Information from 232 wells has been entered into ASCII files; 456 of the 486 total wells were also entered into the UGS-developed INTEGRAL database being used for the Ferron project (table 1). Interpretations (thickness, type of lithology, and geologic description) have been completed for 473 wells.

All base maps have been digitized for the seven 7 1/2' quadrangles within the study area. Also digitized on these base maps are drill-hole locations (petroleum exploratory and development wells, and coal core holes), measured sections, coal outcrops, coal mined-out areas, drainages, and the top and base of the Ferron Sandstone.

Case Studies

Geophysical logs run in the Ivie Creek Nos. 3, 5a, and 9a core holes include the formation density, caliper, and gamma ray (figure 3). Sonic and dipmeter logs were recorded in the Ivie Creek No. 3, the only core hole which was able to hold water. The Ivie Creek No. 9 core hole was abandoned due to problems before coring and logging operations could be completed. Continuous logging of the core recovered from the Ivie Creek Nos. 3, 5a, and 9a core holes was conducted using the a computer-interfaced multisensor track which simultaneously recorded natural gamma, density (via gamma-ray attenuation), and magnetic susceptibility. These data are being used to determine porosity and clay content which are the dominant controls on fluid flow (permeability) in the Ferron Sandstone and most other oil-producing fluvial-deltaic reservoirs.

The No. 1 sandstone parasequence set represents a river-dominated delta deposit which changes from proximal to distal (where the sandstone pinches out) from east to west across the Ivie Creek area. The No. 2 sandstone parasequence set contains more and cleaner sand, indicating a more wave-influenced environment of deposition.

Mini-permeameter Measurements A total of seven permeability transects, four vertical and three sub-horizontal (parallel to bedding), were made on the outcrop at the Ivie Creek case-study site during the 1994 field season (figure 4). The transects as a group sample the proximal, middle, and distal portions of the delta-front rocks of the No. 1 sandstone. Transect locations were designed to encompass most of the lithofacies present in the delta-front sequence. Data from these transects will be used to determine the statistical structure of the spatially variable permeability field within the delta front, to investigate how geological processes control the spatial distribution of permeability, and to evaluate permeability measurement techniques.

During the quarter, mini-permeameter testing in the laboratory was completed for transects T1, T2, T3, T4, T5, T6, and T7 (figure 4). Core plugs obtained from the No. 2 sandstone in the field, and core from the Ivie Creek 5a and 9a core holes were also tested. These permeability data and related information are being entered into spreadsheets for subsequent analysis and transfer to the INTEGRAL database. Several additional core plugs were obtained in the field from the most proximal locations of the No. 1 sandstone east of the mini-permeameter transects at the Ivie Creek site. These samples will be also tested for comparison to the other portions of the delta-front rocks.

The results of mini-permeameter tests performed on core collected from vertical (T1, T2, T3, and T4) and horizontal (T5, T6, and T7) outcrop transects are shown on figures 5, 6, and 7. A large percentage of the rock tested apparently has a permeability lower than the resolution of the mini-permeameter (approximately 2 millidarcies [md]). Overall, permeabilities in the No. 1 sandstone are relatively low, less than about 50 md. In the No. 2 sandstone (figure 6), permeabilities are locally much higher, in excess of 80 md. A clear increase in permeability within distinct bedforms of the No. 1 sandstone exists from distal to proximal transects (T2 to T1 to T3 to T4). Although permeability values are below instrument resolution in horizontal transects T6 and T7 (figure 6), results obtained from T5 suggest that there is a definite permeability structure that may correspond to mappable variations in lithology and grain size.

In performing the field and laboratory testing, we collected sufficient information to compare the results of in situ testing (k-hole) to laboratory tests performed on core (k-plugs) collected from the holes tested in the field (figure 8). The in situ tests produce generally larger permeability values. We suspect that this effect might result from differences in surface preparation; the core plugs are trimmed with a saw, while testing surfaces in the in situ holes are chipped to a roughly flat surface. Because in situ testing requires a large field commitment (in time and personnel) and appears to provide overestimates of rock permeability, we will continue to emphasize the collection and laboratory testing of core plugs.

Outcrop core-plug samples of both the Nos. 1 and 2 sandstones in the Ivie Creek case-study site were collected during the 1994 field season to characterize the vertical and lateral variations of petrophysical properties such as density, velocity, mineralogy, and porosity. These samples were also tested for permeability (figure 9). Figure 9 shows that at least 14 percent porosity is required to obtain measurable permeabilities with the mini-permeameter. Data shown in figure 9a are grouped to illustrate that grain-size variations appear not to influence the relationship between permeability and porosity. Similarly, data shown in figure 9b indicate that permeability-porosity relationships are comparable for both the Nos. 1 and 2 sandstones.

Detailed mini-permeameter testing is being performed using Mobil Oil Corporation's stage-mounted automated mini-permeameter. Permeability data are being collected at 0.05 foot (1.5 cm) intervals along the core recovered from the Ivie Creek drilling operations. Figure 10 illustrates low permeabilities obtained from two core segments from the Ivie Creek 5a (No. 2 sandstone) and 9a (No. 1 sandstone) core holes. Overall, the results are similar to those obtained from outcrop-derived core plugs.

Reservoir Modeling

TECHNOLOGY TRANSFER

"Response of Delta Morphology and Progradational Style to Changes in Accommodation, Sedimentation, and Basin Topography: Ferron Sandstone, East-Central Utah" by R.D. Adams and F.W. Stapor; Geological Society of America Annual Meeting, Seattle, Washington, October, 1994.

"Influence of Tectonics, Sea Level, and Basin Topography on the Geometry of a Deltaic System: Ferron Sandstone, East-Central Utah", by R.D. Adams; Distinguished Lecture Series, University of Utah Department of Geology and Geophysics, November, 1994.