Neil F. Hurley

Quantification of Vuggy Porosity in a Dolomite Reservoir from Borehole Images and Core, Dagger Draw Field, New Mexico

Vugs, which are large irregular pores, are a key pore type in many carbonate reservoirs. Because of coarse sample rate and limited vertical resolution, conventional wireline logs commonly mask vuggy porosity. This becomes a major problem when logging tools combine high vuggy porosity with low matrix porosity to derive average porosity values that fall below arbitrary commercial cutoffs. Non-completion of such zones commonly results in bypassed pay. Borehole-imaging logs are high-resolution, oriented electronic pictures of the wellbore made by electrical or acoustic devices. Pixel-counting techniques, when applied to borehole images, can be used to detect and quantify vugs. The purpose of this study is to calibrate vugs quantified from borehole images to those quantified from digitally scanned core. Samples studied are vuggy dolomites of the Cisco and Canyon Formations (Pennsylvanian), Dagger Draw field, Eddy County, New Mexico. Results suggest that the neutron log is a good total-porosity device. However, this log suppresses the evidence for vuggy porosity which can lead to bypassed pay.

Pore-Size Distributions in Vuggy Carbonates From Core Images, NMR, and Capillary Pressure

Vugs, which are defined as pores larger than adjacent grains, are common in many carbonate reservoirs. Such pores are not always recognized by conventional wireline logs. The purpose of this study is to compare pore-size distributions in Oligocene carbonates from China using core, borehole images, nuclear magnetic resonance, and capillary pressure. The Kerr-McGee CFD2-1-2 well was drilled in the Bohai Basin in China. The reservoir consists of dolomitic limestones and fractured dolomite. The core (25 meters) was polished and inked with fluorescent paint, then photographed under black light to generate high-resolution, 2-D black and white photos. Image analysis produced detailed information about vug size and depth. Core images were then used to calibrate a Formation MicroImager (FMI) log, so that pixel counts of vugs from the FMI would match core observations. Nuclear magnetic resonance (NMR) logs measure the T2 relaxation time, a parameter that is a function of pore size and reservoir fluid composition. Laboratory NMR measurements for six core plugs have been compared to pore-size distributions from core images. These distributions are comparable for high T2s (above 92 milliseconds). Mercury intrusion capillary pressure (MICP) measurements are used to determine pore-throat size distributions. These distributions, which can be directly compared to NMR and image analysis pore-size distributions, have very similar shapes. The net result of this study is that a technique has been developed to relate core-calibrated borehole images to NMR and MICP data. This is an important step in the difficult process of understanding NMR log behavior in vuggy carbonate rocks.

Dual-lateral horizontal wells successfully target bypassed pay in the San Andres Formation, Vacuum field, New Mexico

This case study of the San Andres Formation in the mature Vacuum field, New Mexico, shows how seismic data can be used to target bypassed pay with horizontal wells. These dual-lateral wells were the first attempt at horizontal development in the Vacuum San Andres field and in the San Andres Formation in New Mexico. The primary reservoir facies consist of ramp crest and outer ramp dolomitized peloidal packstones, skeletal and ooid grainstones, and fusulinid packstones. Vertical facies successions form numerous high-frequency carbonate depositional cycles and cycle sets that create distinct reservoir zones. Structural blocks created by small-scale faults (25 ft [8 m] vertical displacement) and bypassed pay located in thin depositional cycles were identified with three-dimensional compressional-wave seismic amplitude and coherency volumes and well data and targeted using medium-radius horizontal wells. Horizontal wells penetrated fault blocks and depositional cycles that were not adequately drained by existing vertical wells.Production curves show a significant increase in production from the horizontal wells and no interference with production from offset vertical wells. This suggests that the faults are partially sealing.

Using Borehole Images for Target-Zone Evaluation in Horizontal Wells

Horizontal wells are rarely horizontal. Instead, operators commonly try to drill such wells into particular rock layers, or target zones, which may or may not be truly horizontal. Thicknesses of target zones commonly range from a few feet to a few tens of feet (1-10 m). Target-zone evaluation concerns whether a horizontal well was successfully located and drilled in a given rock layer. Borehole-imaging logs provide a powerful tool for stratigraphic interpretation and target-zone evaluation in the Austin Chalk, Niobrara Formation, San Andres Formation, and other units. This study uses borehole images generated by Schlumbergers Formation MicroScanner (FMS), a microconductivity logging device. Open fractures and clay-rich interbeds appear as dark, high-conductivity traces on the FMS log. These traces can be fit with sinusoidal curves and oriented on a computer workstation. The shape of the sinusoidal curve that fits a particular bedding plane tells the interpreter whether the borehole was moving upward or downward through the strata. STRATLOG (trademark of Sierra Geophysics, Inc., a Halliburton Company) software has been used to display borehole profiles by combining FMS data on fracture intensities and bedding-plane intersections with gamma-ray logs, mud logs, and borehole-deviation surveys. To aid in planning future wells, multiple penetrations of he same horizon can be detected and used to calculate highly accurate bedding-plane dips. Fault interpretation, including the detection of rollover beds, is also possible. Finally, stratigraphic interpretation can be combined with observed fractures to determine which rock layers are most highly fractured, and, therefore, should be target zones.

Anhydrite distribution within a shelf-margin carbonate reservoir:San Andres Formation, Vacuum Field, New Mexico, USA

Anhydrite cement causes significant heterogeneity within the San Andres reservoir of the Vacuum Field where it is associated with faults, fractures, karst zones and highly cemented dolomite intervals. The primary reservoir rocks within the Central Vacuum Unit of the Vacuum Field are dolomitized peloidal packstones, skeletal and ooid grainstones and fusulinid packstones. These rocks alternate with lower reservoir-quality dolomite intervals with variable amounts of anhydrite cement. Nodular and pore-filling fabric-selective anhydrite cements are common within the reservoir interval. Cross-plots of apparent matrix grain-density versus apparent matrix volumetric cross-section (ρmaa–Umaa cross-plots), combined with Vp/Vs seismic attributes and core data, provide insight into the vertical and lateral distribution of anhydrite within the San Andres reservoir. Anhydrite is generally concentrated in thin depositional cycles or intervals that are separated by relatively anhydrite-free cycles that exhibit relatively higher porosity and permeability. Lower Vp/Vs values correspond to higher percentages of anhydrite and are useful formapping anhydrite distribution. This evaluation of anhydrite distribution providesan estimate of the significant cementation-related heterogeneities within the reservoir that is useful for development-well planning and to target areas of bypassed pay.

Incremental oil recovery using a horizontal drainhole in the San Andres Formation, Olson field, west Texas

Oolitic carbonates and associated rocks in the San Andres Formation produce hydrocarbons at Olson field, west Texas. Mudstones, dolomitized fusulinid-peloid packstones and wackestones, ooid-peloid grainstones and packstones, and minor siliciclastic sediments occur in the field. The reservoir is stratigraphically compartmentalized by lenticular oolite deposits that are overlain by algal-laminated mudstones and siltstones. Pore-filling anhydrite cement, common in all lithologies, adds to porosity heterogeneity. Interwell communication is poor, as indicated by variable bottomhole pressures and erratic waterflood response.A medium-radius lateral hole was drilled to a total depth approximately 50 ft (15 m) from an offset vertical well. At this depth, the horizontal well intersected a previously existing artificial fracture that occurred in the nearby vertical well. The intersection of this fracture by the lateral borehole had a significant economic impact. The production rate in the vertical well jumped from a few barrels per day to an average of 70 BOPD and less than 20 BWPD. The ensuing years maintained relatively high flow rates. Projected incremental oil recovery is 153,000 bbl, roughly equivalent to production from an average well drilled during the early life of the field.Other unsuccessful horizontal wells have been drilled at Olson field, but they have not targeted preexisting hydrofractures in offset vertical wells. Therefore, the concepts presented in this paper have not been retested. This approach, intentionally drilling a drainhole to intersect a preexisting hydrofracture, could add new life to many older, compartmentalized reservoirs.

Turbidite Outcrop 3-D Ground Penetrating Radar Imaging: Lewis Shale, WY

Development of deepwater turbidite reservoirs is an extremely costly undertaking. Because many of the geological features likely to control production from turbidite reservoirs are smaller that the resolution possible from even the highest quality ocean bottom cable 3-D seismic reflection surveys, it is necessary to examine equivalent outcrops on land where they are accessible to inspection. The characterization of such outcrop analogs by means of geological mapping, photomosaics, and behind outcrop coring and logging has helped lead to new deepwater discoveries, and programs for developing catalogs of digital outcrop models are presently being proposed for this purpose.

Platform-Margin and Marginal Slope Relationships and Sedimentation in Devonian Reef Complexes of Canning Basin, Western Australia: ABSTRACT

Devonian limestone platforms in the Canning basin were generally rimmed by reef-margin and reef-flat deposits, constructed by stromatoporoids, algae, and corals in the Givetian and Frasnian, and by algae in the Famennian. However, some platforms were low-relief banks with little or no reef development. End_Page 516------------------------------ Reef rims were flanked by steeply dipping marginal-slope deposits that descended to depths up to several hundred meters. These deposits include debris flows, huge allochthonous reef blocks, scheck breccias, and reefal tongues and bioherms built primarily by sponges and algae. Platform margins are classified as advancing, retreating, upright backstepping, and roll-over types. Advancing margins are typical of the Famennian reef complexes; the others occur principally in the Frasnian and Givetian, where they are often associated with platform-margin unconformities resulting from submarine erosion or margin collapse. The reefs and slowly deposited parts of the marginal-slope facies were subject to pervasive early submarine cementation by fibrous high-magnesium calcite (now radiaxial spar). The strongly cemented reef limestones formed rigid wave-resistant rims to the platforms. Fracturing of these limestones, probably largely associated with earthquake shaking, gave rise to extensive networks of neptunian dikes and sills, and to the collapse of some sections of the margins. Such collapses in turn initiated debris flows and the deposition of allochthonous reef blocks on the adjoining marginal slopes. The reef complexes are being explored extensively for lead-zinc deposits in outcrop and oil in the subsurface. A significant oil discovery was made in a Famennian platform margin (the Blina field) in 1981. End_of_Article - Last_Page 517------------

Seaward Primary Dip of Fall-in Beds, Lower Seven Rivers Formation (Permian), Guadalupe Mountains, New Mexico: ABSTRACT

Fall-in beds are shelf carbonate rocks which exist adjacent to the Capitan Limestone in a belt about 1 km wide, and which have basinward dips of 5 to 15°. Sedimentologic and structural-geopetal data gathered in field studies of the lower Seven Rivers Formation in North McKittrick Canyon show that tectonic tilting and/or compactional subsidence can account for only part of the basinward dip of fall-in beds, the remainder being primary depositional dip. The dominant lithologies of fall-in beds are stromatolitic algal oncolite rudites and sand-sized, mixed skeletal-peloid grainstones. Rocks are tightly cemented with marine phreatic isopachous fibrous magnesium calcite. Fall-in beds lack features of the adjacent, shallower, but generally submerged shelf-crest facies such as fenestral fabric, pisolites, tepees, erosion surfaces, and shoaling cycles. An inferred energetic, subtidal marine depositional environment for fall-in beds is compatible with their significant basinward depositional dip. Primary geopetal fabrics, although scarce in fall-in beds, have dips not exceeding a few degrees. The dip divergence between bedding planes and geopetal surfaces averages 8 ± 2°, a value which is inferred to be equal to the original depositional dip. Proof of primary seaward dip in fall-in beds lends support to Dunhams marginal-mound hypothesis for the Capitan shelf. Also, primary dip in beds adjacent to the Capitan supports recent interpretations that the Capitan Limestone formed in a relatively deep (30 to 50 m), continually submerged shelf-edge position, and was not a true barrier reef. End_of_Article - Last_Page 471------------

Geology of Oscar Range (Devonian) Reef Complex, Canning Basin, Western Australia: ABSTRACT

The Oscar Range is a Late Devonian reef complex that formed at the margin between the Precambrian Kimberley craton and the Canning basin. The range covers an area of 80 × 10 km (50 × 6 mi), and resembles a large atoll in that Frasnian and Famennian reef, marginal-slope, and back-reef subfacies grew around an exposed Precambrian core. Frasnian reefs are dominated by stromatoporoids and Renalcis, and the reefs show periods of upbuilding, drowning, backstepping, and basinward progradation. Fault-controlled reef growth is developed locally. Marginal-slope deposits contain stromatoporoid debris, sponge boundstone, and crinoids. Back-reef deposits are generally Amphipora-rich biostromes, although fenestral oolite-intraclast-peloid beds are widespread in the southern Oscar Range. Dolomite is best developed in Frasnian back reef and, to a lesser extent, in reef-margin and reef-flat subfacies. In the area of the Precambrian core, conglomerates of Precambrian debris are interbedded with Frasnian limestones that are in depositional contact with basement. Hills of Precambrian rocks rise tens of meters above these peritidal ba k-reef deposits, indicating perhaps several hundred meters relief during the Frasnian. The exposed Frasnian-Famennian contact is a disconformity. Famennian reefs are dominated by Renalcis and associated algal stromatolites; the equivalent marginal-slope is characterized by allochthonous reef blocks, sponge bioherms, and crinoidal debris. Steeply dipping (40°) basinward-prograding reef and marginal-slope tongues make up the shelf margin. The Famennian back reef is composed of fenestral oolite-peloid lithologic units with common teepee structures, flat-pebble conglomerates, and cryptalgal fabrics. End_of_Article - Last_Page 490------------