Super-permeability or super-k (as defined here), refers to confined intervals that have production or injection rates of at least 500 barrels of fluid/day/foot. Eight cored wells—four producers and four injectors—show super-k performance within the northern Hawiyah study area. One injector well had multiple super-k flow intervals. Super-k flow correlates with specific limestone, dolomite, or fractured intervals based on an analysis of core and flowmeter data. A thin, high-permeability unit sandwiched between low-permeability strata characterizes the sequential stratification of stratiform super-k flow units. Oolitic, mixed skeletal pelletoidal, foraminiferal, fragmented Cladocoropsis or Cladocoropsis lithofacies may make up the high-permeability conduits in limestones, and various mud-dominated facies form the tight enclosing layers. Sucrosic or vuggy fabrics characterize the highly permeable layers in dolomite-controlled intervals, and mosaic textures form the tight envelope. Production from stratiform dolomite is typically from one or more thin (generally less than six-inch thick) stringers some of which represent tempestite deposits. Super-k flow from fracture-controlled intervals has no correlation with either facies or dolomite textural boundaries. An unexpected result of this study was the discovery that not all super-k flow comes from high-permeability features such as fractures or zones of dolomitized leached Cladocoropsis. Instead, ordinary rock fabrics with normal permeability ranges (0.1 to 1 darcy) characterize most of the super-k intervals in six of the eight wells examined.
In general terms, super-permeability (super-k) encompasses all unpredictable and anomalous rates of production or injection from confined intervals that severely challenge reservoir management. Its use in this paper is more specific whereby super-k behavior refers only to intervals whose production or injection rate equals or exceeds 500 barrels of fluid per day per foot (bf/d/ft). Although not all encompassing, this definition is unambiguous and can be measured by a flowmeter.
At its most severe, super-k flow contributes to water overriding oil, bypassing oil, and the premature watering-out of wells (A.S. Al-Muhaish, Saudi Aramco, oral communication, 1997). Anomalous high rates of flow from confined intervals are a well-known phenomena in many reservoirs including the Arab-D of the Ghawar field (Moore, 1989; Valle et al., 1993).
The Arab-D in the Hawiyah area (Figure 1) of the Ghawar field was chosen for a study of super-k flow. An analysis of production tests and logs confirmed that the phenomenon of super-k production and injection is widespread in this area. Various highly permeable geologic features such as fractures, and karstic or dolomite diagenetic overprints, were known or suspected causes and controls on the distribution of super-k in the Arab-D (Moore, 1989; Valle et al., 1993). In particular, the leaching and dolomitization of Cladocoropsis (a stick-like stromatoporoid) appeared to be especially important (Moore, 1989; Valle et al., 1993) for super-k flow in the Uthmaniyah area.
An understanding of super-k is important, as its abnormal properties present major problems in reservoir management. Attempts at developing creative solutions for super-k problems have been largely premature because the stratigraphic characteristics of these highly permeable intervals are poorly known. As there are few detailed lithologic descriptions for super-k strata in the Ghawar field, the chances of developing predictive super-k models are greatly reduced. Although not the only control, a thorough stratigraphic description represents an initial step in developing a predictive super-k model. This is the case in the Hawiyah area with its abundance of super-k intervals. Moore (1989) discussed the prediction of super-k, but the geometry of geobodies is too poorly known to be able to do this without flowmeter confirmation.
Limitations in our stratigraphic understanding of super-k intervals are primarily due to perceived poor core recover. Consequently, the description of core from most super-k intervals has been given only a cursory consideration. Loss of core is less of a problem in Hawiyah than generally believed, as is shown by the number of wells that have core from Arab-D strata correlative with established super-k zones. Hawiyah wells with cored super-k zones present an excellent opportunity to characterize the geology of strata that supports super-k flow rates in the Arab-D, and in the Hawiyah area specifically. This paper presents the results of a stratigraphic and petrophysical analysis of cored super-k intervals in eight wells (HWYH-A to H) from the Hawiyah area of the Ghawar field (Figure 1). Special attention was given to the identification of fractures in all cores and especially in intersections of super-k intervals, as they are believed to exert an overriding influence on the occurrence of super-k. This study also relates the distribution of cored super-k intervals to the existing reservoir zonation and thereby to a lithofacies-driven stratigraphic framework.
MATERIALS AND METHODS
An examination of core plugs, and of slabbed core lightly etched with 10 percent hydrochloric acid, formed the basis for the core descriptions. Although photographs showing fragmented and poorly fitting core pieces may suggest otherwise, most wells in this study had full core recovery over the super-k intervals. Exceptions include core from wells HWYH-C, G and H. Minor loss off the ends of individual core pieces followed destructive core plugging, but this generally amounts to no more than a fraction of an inch. Cores do not differ in length significantly from the original recovery and only slightly from the length actually placed in each core box. An audit of all core-plug holes identified missing core greater than a few inches. Even with highly fragmented core, this process identified small missing core lengths because of the six-inch spacing for most drilled vertical and horizontal core plugs.
The observed core discontinuity is not so much a core-loss problem as an expression of how cores were slabbed. All cores examined were cut into three parts along their length before establishing a proper fitting between successive core pieces. Core boxes currently contain only one of the three disjointed core slabs, a condition that accentuates the erroneously perception of significant missing core.
The process of depth-matching core provides corroborative support that little of the core is missing. This procedure matches the curve for measured core-plug porosity with the trace of in situ porosity obtained from either the neutron or bulk density logs. The core log is depth shifted by virtue of being linked to the measured core-plug porosity (Figure 2). Very fine control is possible in depth-shifted core descriptions because the typical six-inch sample spacing of core plugs correlates with the sampling frequency of porosity well logs. Measured porosity traces for a single core were not stretched to fit with the porosity log trace, instead the core was split into two or more segments to establish the best depth-match as shown in Figure 2. Fragmented core and places where core pieces have worn or ground ends (spun core) establish the location of the split and thereby the missing core (Figure 2). Note that the scale of the lithologic logs used makes it impractical to show intervals of missing core less than a few inches thick.
The use of core plugs from Arab-D reservoir rocks has several advantages over descriptions based only on oil-stained, slabbed core. Most importantly, core plugs provide an excellent opportunity to quantify and tie rock fabrics to petrophysical measurements. Depositional texture recognition is also simplified because petrophysical analysis involves the routine removal of hydrocarbons stains that tend to obscure rock-fabric relationships in oil-stained core slabs. Finally, the petrophysical linking of rock fabrics and porosity measurements provides a convenient and accurate means of depth-matching the core with the neutron logs or bulk density logs.
All core/core-plug descriptions included a microscopic assessment of Dunham textural types, average grain size, percentage dolomite, percentage anhydrite, major grain types, and percentage isolated pore space present in core plugs. Thin-section analysis supplemented the rock-fabric descriptions and provided the basis for characterizing the pore system from intervals correlated with super-k flow (Table 1). The lithofacies, primary sedimentary structures, biogenic structures, bedding and diagenetic boundaries were classified from an examination of the slabbed core (see Figure 3).
|Well HWYH||Core no.||Depth (ft)||Plug no.||PO||Porosity (%)||K (md)||Percent||Texture Code||Grain Size||Litho-facies|
|Well HWYH||Core no.||Depth (ft)||Plug no.||PO||Porosity (%)||K (md)||Percent||Texture Code||Grain Size||Litho-facies|
PO = plug orientation, H = horizontal, V = vertical. Ls = limestone; Dol = dolomite.
Texture Code (modified Dunham Classification): pk = packstone; mlpk = mud-lean packstone; gr = grainstone; dol = dolomite. Grain size = Wentworth scale. Lithofacies type: str = stromatoporoid; bur = burrowed; clad = Cladocoropsis; mud = muddy fragmented Cladocoropsis; frag = fragmented Cladocoropsis; MSP = mixed skeletel pelletoid; ool = oolitic; xxx = unknown.
RESERVOIR STRATIGRAPHY AND ZONATION
Figure 3 is a generalized stratigraphic sequence and reservoir zonation for the Arab-D in the Hawiyah area of the Ghawar field. The reservoir section depicts a thick sedimentary package with a composite cyclical arrangement of grain-dominated and mud-dominated carbonates. Overall, the Arab-D reservoir represents part of a major upward-shoaling cycle whose lithofacies becomes more grain-dominated and porous toward the top. Repetitive textural and porosity stacking patterns suggest that several intermediate- and small-scale depositional cycles make up the major upward-shallowing cycles.
Porosity well-log patterns express the cyclical deposition by a succession of porosity breaks. Saudi Aramco geologists recognize six widely correlative porosity log breaks that subdivide the reservoir into five zones (Figure 3). Except for Zones 1 and 2, Saudi Aramco tentatively regards the zone boundaries as time lines that separate field-wide depositional packages (Mitchell et al., 1988). Zones 1 and 2 are time-transgressive units that follow the lateral facies migration in the upper reservoir section.
STRATIGRAPHIC AND GEOGRAPHIC DISTRIBUTION OF SUPER-K
Flowmeter analysis of 240 wells in the Hawiyah area identified 88 producers and injectors with super-k flow rates. Only 17 of the 88 wells cored the super-k intervals. This study focused on eight cored wells from the northern part of the Hawiyah area (Figure 1) where ongoing field development has concentrated attention on super-k related problems. Half of the selected super-k wells are injectors, three of them being located on the eastern flank of the area and one on its western flank.
Figure 1 shows the distribution of super-k occurrences in relation to reservoir zones. The greatest concentration of wells with super-k flow (59) is in reservoir Zone 2B. The next most abundant zone is Zone 2A containing 24 wells with super-k flow. Super-k in these zones coincides with stratal units typically dominated by grain-supported depositional fabrics (Meyer et al., 1996). Only a few examples of super-k flow occur in reservoir Zone 3, and these are from intervals near the top of the zone.
PETROGRAPHIC AND PETROPHYSICAL DESCRIPTIONS OF SUPER-K INTERVALS
Petrographic and petrophysical examinations were made of the core from the eight Hawiyah area wells (HWYH-A to H) (Figures 4 to 11). The cores were slabbed and thin-sections made from samples of the super-k intervals. In Figures 4 to 11, it should be noted that (1) the blue material in the thin-section photomicrographs is plastic that impregnates and defines pore spaces, and (2) core trays are 3 ft long.
Wells HWYH-A, HWYH-B, and HWYH-C contain super-k intervals that occur exclusively in limestone, whereas wells HWYH-D, HWYH-E, HWYH-F, and HWYH-G flow from dolomite sections. Well HWYH-H has super-k flow that correlates with a limestone and dolomite succession.
An open-hole flowmeter run identified super-k flow from a single zone (6,881–6,885 feet (ft)) in HWYH-A (Figure 4A). The depth indicates that the flow is from a grainstone located between mud-dominated strata (Figure 4B). The grain size changes across a scoured surface to distinguish two beds within the grainstone unit. Each bed has a grain-supported framework made up of a moderately sorted skeletal grain assemblage. This carbonate sand, composed mainly of coarse, subrounded Cladocoropsis fragments (Figures 4C and 4D), identifies these sediments as belonging to the fragmented Cladocoropsis lithofacies (Meyer and Price, 1993).
Thin-section petrography reveals a framework of skeletal grains that supports a system of pores dominated by interparticle and intraparticle porosity (Figures 4C and 4D). Porosity occlusion by cementation is a minor factor. Very thin and poorly developed pore-lining calcite cement adds to the irregular geometry of pores. Locally, syntaxial overgrowths partly or totally occlude the pore space around echinoderm fragments (Figure 4D).
Completely churned to strongly burrowed mottled mudstone and wackestone textures make up the strata above and below the grainstone layer. These mud-dominated rocks have interparticle microporosity and a significant fraction of fossil-selective moldic porosity or separate vug porosity as defined by Lucia (1983).
HWYH-B is a high-rate (42,338 barrels of water per day [bw/d]) injector well. Cumulative flowmeter profiles record six separate intervals (A to F) that have super-k properties. Collectively, they account for about 75 percent of the total volume of water injected into this well. All six intervals are in limestone, but they consist of eight lithofacies (Figure 5A).
Six percent (2,540 bw/d at a rate of 500 barrels of water per day per foot [bw/d/ft]) of the injected water enters the reservoir between 7,365–7,370 ft, the stratigraphically highest super-k interval (super-k A). This interval equates with a cored 4.5-foot section of mostly clean oolitic grainstones. Below it is a foot-thick lime mudstone and above is a thick, mud-dominated succession of lime packstone identified as part of the mixed skeletal pelletoidal lithofacies (Meyer and Price, 1993). Petrophysically, the super-k grainstone supports a heterogeneous system of well-connected interparticle pores together with some poorly connected moldic porosity. Permeability core plug measurements reflect this variability. Not all grainstones within this interval have good petrophysical characteristics of permeability and porosity. Low porosity and permeability characterize two hardgrounds and several thin, muddy beds less than six-inches thick. In the hardgrounds, intense cementation along the surface has sealed the interparticle pore spaces. Collectively, the poor reservoir units interrupt the vertical continuity of the oolitic succession.
Another five percent of the injected water (2,117 bw/d) passes into the reservoir at super-k rates (705 bw/d/ft) over an interval of three feet (super-k B; 7,371–7,374 ft). A one-foot-thick, tight mudstone separates super-k B from super-k A. As in the oolitic super-k A, the flow occurs in clean grainstones, but it differs in having a relatively uniform rock fabric of the mixed skeletal pelletoidal lithofacies. The pelletoid-dominated grainstones support mainly interparticle porosity, but some poorly connected moldic porosity occurs in a thin layer toward the base of the super-k interval. A permeability measurement of 23 millidarcies (md) characterizes the layer in which moldic pore space predominates, whereas measurements in the range of 300 md to more than 1 darcy characterize rock fabrics dominated by interparticle pore spaces.
The interval from 7,376 to 7,382 ft corresponds to super-k C and accepts 27 percent of the injected water (Figure 5A). Volumetrically, the raw flowmeter data suggests that this is the most important zone, but it does not possess the highest rate of flow on a per-foot basis. Unlike the two previously described super-k intervals, combinations of three lithofacies (oolitic, mixed skeletal pelletoidal, and foraminiferal) correlate with this high-permeability zone. These lithofacies make up an amalgamated succession of grain-dominated carbonates whose rock fabrics and pore networks are very similar to those described previously for the oolitic and mixed skeletal pelletoidal lithofacies. An increased amount of intraparticle foraminiferal porosity represents the only significant difference between the pore networks of the foraminiferal and mixed skeletal pelletoidal lithofacies.
Super-k D (7,386.5–7,390 ft) has the highest rate of injection of just over 2,540 bw/d/ft. It occurs five feet below super-k C. The highly permeable unit is a grainstone (Figure 5B) composed of cobble-sized Cladocoropsis ‘fingers’, fragmented Cladocoropsis (Figures 5C), and scattered leached coral (Figure 5D). Although this rock fabric consists of much poorly connected pore space within the skeletal fragments, the large grain size creates a framework with large pore throats.
Another interval where water ingress is at super-k rates (931 bw/d/ft) occurs between 7,427–7,432 ft (super-k E). A collection of Cladocoropsis ‘fingers’ and fragmented Cladocoropsis form a pebble to coarse sand-sized textural framework. A progressive decrease in grain size from pebble to coarse sand forms two graded beds in the super-k interval. The existing framework averages 25 percent porosity and 450 md of permeability. Intraparticle pores in Cladocoropsis skeletal elements account for most of the porosity (14 percent) with the remainder distributed between particles. The abundance of Cladocoropsis ‘fingers’ in each of these two graded beds warrants their placement in the Cladocoropsis lithofacies.
The deepest of the six super-k intervals (super-k F; 7,445–7,450 ft) is similar to super-k zones A and B both in thickness (5 ft) and rate of flow (677 bw/d/ft). Three lithofacies are present in this unfractured interval (Figure 5A). They represent poor reservoir rocks whose petrophysical properties are inconsistent with the observed super-k flow. Pore throats are only a few tens of microns in diameter and permeability averages less than one millidarcy.
This discrepancy between reservoir rock quality and flow behavior in super-k F is an unresolved problem. The good match between the measured porosity curve and the formation density logs and compensated neutron logs implies that the mud-dominated lithofacies are logged correctly. Rocks whose petrophysical properties are more consistent with the super-k injection rate occur five feet higher stratigraphically. The temptation is to question the depth indicated by the flowmeter and to suggest that it needs correction.
HWYH-C has a single interval of super-k flow between 6,050 ft and 6,056 ft. The super-k flow interval is a stacked succession of three normally graded beds (Figure 6A), each ranging in thickness from 1.5 to 2 ft. Each bed begins with a grainstone and grades upward into a packstone through an interval of mud-lean packstone (Figure 6B). The average grain size of the graded beds ranges from medium to fine sand. Grainstone, made up mainly of rounded, equant to elongated pelletoids and locally occurring grains that superficial resemble ooids, is the most abundant texture. Grain assemblages formed chiefly of pelletoids, and lesser amounts of forams and echinoderm fragments, characterize the mixed skeletal pelletoidal lithofacies (Figures 6C and 6D). This lithofacies makes up the basal and top beds of the lower interval. Superficial ooids predominate in the middle bed and represent a significant component of an oolitic lithofacies. These grainstone frameworks of medium to lower-medium sand-sized grains support a system of interparticle pores and separate vug porosity. The subparallel arrangement of elongated grains locally produces millimeter-scale laminations and a directional porosity network (Figure 6D). Hairline fractures in the packstone (recognized by minor differences in hydrocarbon staining) augment variations in the matrix pore network in the basal part of the super-k interval (Figure 6B). Two hairline fractures occur within the packstone. The fractures measure a few tens of microns in width, are open, and nearly vertical. A coral rubble zone makes up the underlying three feet of core.
Porosity measurements made on horizontal and vertical core plugs show little variation in the high pore volume throughout the super-k interval (Table 1). The corresponding permeability values are moderate (87–637 md), whereas multi-darcy-scale permeability would seem more appropriate for an interval with a super-k production rate. However, it is possible that the petrophysical measurements failed to capture the effect of the fracture porosity that could be controlling the super-k flow.
A continuous flowmeter log shows that nearly 58 percent of injected water enters a two-foot interval from 7,695 ft to 7,697 ft (Figure 7A). Examination of the core suggests that the super-k flow correlates with thin porous and permeable dolomite at the same depth (Figure 7B). This vuggy dolomite consists of a framework of medium-sized dolomite rhombs that support a system of connecting intercrystal and vuggy porosity (Figure 7C). Vugs (average diameter four millimeters) have an uneven distribution, and make up only a small percentage of the total pore system. Though volumetrically minor, the vugs cluster in several thin zones. Such distributions result in touching-vugs with channel-like features.
Plugs taken for porosity and permeability measurements (Table 1) missed the touching-vug parts of the super-k interval. Even so, the measured porosity averages nearly 30 percent and the average permeability exceeds 2,000 md for the super-k interval.
Super-k production comes from the base of a thick dolomite interval at 6,186 to 6,189 ft in HWYH-E (Figure 8A). The texture varies from a crystalline mosaic to a crystalline sucrosic dolomite and includes two thin vuggy intervals (Figures 8B). Both dolomite textures have a framework formed by fine dolomite rhombs with cloudy centers, but their porosity structure differs significantly. Sucrosic dolomite (Figure 8C) has a well-connected intercrystalline and vuggy pore system. In contrast, mosaic dolomite (Figure 8D), which makes up most of the cored super-k interval, features a system of pores limited largely to micro intercrystalline spaces.
Porosity and permeability measurements (generally below 10 percent and 1 md respectively) reflect the poor reservoir quality of mosaic dolomite. Improved reservoir qualities occur within sucrosic and vuggy dolomite intervals found only in thin stringers near the top and bottom of the super-k interval. Two sucrosic dolomite stringers at the top of the super-k interval have porosity measurements greater than 20 percent, but permeability readings barely exceed 100 md. A vuggy dolomite stringer near the base of the super-k zone has a low concentration of isolated elongated vugs, and porosity and permeability values that are only slightly better than values from the mosaic dolomite. There is no evidence of fractures.
In well HWYH-F, super-k flow (6,528–6,531 ft) is also from dolomite (Figure 9A). Outwardly, the core from this super-k interval (Figure 9B) resembles that from HWYH-E (see Figure 8B). The alternating sucrosic and mosaic texture of the dolomite differs only in crystal and pore size and the textural pattern is similar to that of graded limestone beds. Medium-sized dolomite rhombs create framework crystals in each textural type (Figures 9C and 9D). A mosaic texture with low porosity and permeability characteristics dominates the interval but porous sucrosic and vuggy dolomite textures are present in two five- to six-inch-thick stringers (Figure 9B). One stringer occurs at the base and the other near the middle of the super-k interval and both have vugs (possibly unconnected) concentrated at their base. The vuggy intervals pass upward through sucrosic dolomite into a mosaic texture. The vugs represent a minor but important component of the pore system. Their average diameter is five millimeters but at least one vug penetrates the whole core, and resembles a channel. No petrophysical measurements are available from this part of the super-k interval.
Super-k flow occurs between 7,490 ft and 7,500 ft in well HWYH-G. It correlates with dolomitized stromatoporoid and burrowed lithofacies within a massive dolomite sequence (Figure 10A). There was no core recovery from the upper part of the super-k interval. The core is of uniform appearance and in only a few places is the dolomite fabric disrupted by vugs (Figure 10B). In thin-section, the apparently uniform dolomite has a sutured crystalline texture of variably sized dolomite crystals and jagged crystal contacts (Figure 10C). This diagenetic overprint has destroyed nearly all vestiges of the original depositional texture. An exception occurs in the lower part of the interval where there are stromatoporoid structures.
The sutured dolomite has low visual porosity and low to moderate measured permeability (13 percent and 0.8–340 md, respectively). The complex porosity network of the matrix is made up of micro intercrystalline, vug, and intraparticle stromatoporoid pore types. Vugs are as much as three centimeters in diameter but have a scattered distribution. They occur within areas of partly preserved stromatoporoids and probably represent holes formed by fabric-selective dissolution of former stromatoporoids or corals. Scattered vugs and intraparticle stromatoporoid pores conform to the definition of separate vugs that enhance permeability (Lucia, 1983; 1995).
Hairline fractures intersect the dolomite but they are difficult to see in the core (Figure 10B) as they are masked by pervasive tar. Plastic-impregnated thin sections show that these structures form open conduits (Figure 10D) that connect with some of the separate vug and intercrystalline pores. The fractures are also associated with concentrations or lineaments of admixed fine- and medium-sized dolomite crystals. The effect of the hairline fractures on the permeability was not evident from petrophysical measurements on core plugs.
Super-k flow is from a single interval (7,536–7,542 ft) that correlates with a dolomite-to-limestone transition (Figure 11A). Core recovery is incomplete (about one foot is missing) from the super-k flow unit. The missing core is probably part of the muddy fragmented Cladocoropsis lithofacies, an interpretation based on the presence of this lithofacies above the missing section of core.
Two separate segments (7,536–7,538.3 and 7,539.2–7,539.8 ft) make up the recovered limestone portion of the super-k interval. The lower segment passes gradationally into the underlying dolomite (Figure 11A). The upper segment is a moderately permeable (150–1,000 md) fragmented Cladocoropsis lithofacies and the lower one is composed of a low permeability (0–50 md) muddy fragmented Cladocoropsis lithofacies (Figure 11B). The fragmented Cladocoropsis lithofacies is mainly a bedded grainstone made up of a poorly sorted assemblage of fine to coarse Cladocoropsis fragments, intraclasts, pelletoids and other skeletal components. Bedding is defined by scoured surfaces (Figure 11B) and distributional grading. The grainstone framework supports a well-connected but complex system of interparticle pores and some separate vug porosity represented mainly by primary, intraparticle pore spaces (Figure 11C). Complicated grain shapes and minor, pore-lining cement combine to create a complex porosity fabric. Locally, an addition of mud to the interparticle pore space characterizes a packstone fabric in the fragmented Cladocoropsis lithofacies. Complete occlusion of primary, interparticle pore space by lime mud typifies the packstone texture of the muddy fragmented Cladocoropsis lithofacies.
The basal dolomite in the super-k interval consists of a framework of medium-sized dolomite crystals with a sucrosic texture (Figure 11D). The crystal arrangement supports a porosity network of intercrystalline pores and small vugs.
Examinations of core and thin sections did not reveal any evidence of fractures in the super-k interval. Their absence within core immediately above and below the missing core interval strongly suggests that the unrecovered core also lacked fractures.
Framework characteristics suggest that the super-k flow comes from both the limestone and dolomite but it is not uniform throughout the interval. In the absence of fractures, the flow from the limestone is most likely to be strongly fabric-selective and is probably chiefly from the grainstone rather than from the packstone.
STRATIGRAPHY AND OCCURRENCES OF SUPER-K
An analysis of the one-dimensional stratigraphy suggests that flows at super-k rates are from either confined stratigraphic layers (‘stratiform super-k’; Figure 12A) or fractured intervals (‘fracture super-k’; Figure 12B). These two expressions of super-k differ in their relationship to depositional or diagenetic facies. Stratiform super-k flow conforms to specific lithofacies or porous diagenetic dolomite textures whereas the observed fracture super-k flow is not restricted to any individual lithofacies or diagenetic textural boundaries.
Flowmeter analysis and the integration of adjusted core descriptions show that occurrences of super-k flow correlate with six lithofacies in the eight wells examined (Table 1). Specifically, the oolitic, mixed skeletal pelletoidal, foraminiferal, Cladocoropsis, and fragmented Cladocoropsis lithofacies all contribute to super-k flow from limestone, whereas the dolomitized stromatoporoid lithofacies equates with super-k flow in HWYH-G. Three lithofacies—fragmented Cladocoropsis, Cladocoropsis, and mixed skeletal pelletoidal—account for most of the limestone intervals that correlate with super-k injection or production rates.
Stratiform highly permeable flow units are formed by the mixed skeletal pelletoidal and also more frequently, by the oolitic and foraminiferal lithofacies that form units 3.5 ft to 27 ft thick mainly near the top of the reservoir. Combinations of two or more lithofacies may make up some of the super-k zones (Figure 4B), but, typically, only a single lithofacies type is present in a super-k zone. Enclosure of these lithofacies types by tight depositional or diagenetic layers completes formation of the confined flow conduit. The thickness and textures of the confining beds vary, but all possess greatly restricted vertical permeability. Depositional bounding by the burrowed mixed skeletal pelletoidal lithofacies represents the most common configuration and may involve no more than a one-foot-thick interval of tight wackestone, as occurs between super-k A and B in HWYH-B (Figure 5A). In the absence of tight muddy carbonates, intensively cemented horizons such as submarine hardgrounds appear to be sufficient baffles to vertical flow. These diagenetic layers are in most cases only a few inches thick.
Both the fragmented Cladocoropsis and Cladocoropsis lithofacies are stratiform and highly permeable flow conduits that range from one to five feet in thickness. Except for one occurrence where the two lithofacies combine to form a super-k zone, each occurs underlain and overlain by a muddy lithofacies. HWYH-A (Figure 4) provides a good example of a low-permeability burrowed lithofacies that sandwiches a super-k interval composed of the highly permeable fragmented Cladocoropsis lithofacies. In HWYH-B, separate super-k zones formed by the Cladocoropsis lithofacies (Table 1) have the muddy fragmented or burrowed mixed skeletal pelletoidal lithofacies bounding them (Figure 5A). Both of the bounding lithofacies have maximum permeability values of less than 20 md.
Rock Fabrics Associated with Super-k Flow
Few cored super-k intervals from the northern Hawiyah area have the kinds of rock fabrics one might expect to be associated with super-k flow. Measured permeabilities within super-k intervals are good, but few are exceptionally high. Only the dolomite frameworks from super-k flow intervals in HWYH-D and HWYH-H possess pore systems that support permeabilities of as much as 3 darcies. Even so, the flow rate of nearly 9,000 bf/d/ft seems too high for a two-foot-thick interval with an average permeability of 2.7 darcies. Generally, the disparity between the average (and even the highest measured permeability values) and the realized flow rate of a super-k zone is greater than in the previous example and is exemplified by HWYH-F. In this well, 84 percent of the flow (or 5,843 bf/d/ft) comes from a four-foot-thick section. Measurements on core plugs from the dolomite indicate poor permeability (average 120 md; maximum 693 md). How is it that such intervals support super-k flow? One explanation is that subsequent acidizing ‘opened’ up good intervals and made them better than core measurements indicate.
Acid treatment is probably significant in explaining super-k performance in most of the wells. Well records show that one or more episodes of acidification modified the rock fabrics after coring and before the initial flowmeter run in six of the wells. Only HWYH-E and HWYH-F have flowmeter data from wells that were not treated with acid. However, the actual impact of acid treatment on rock fabrics is speculative. No image logs were run in the wells but, even if available, they could provide only limited information about how far the acid invaded or how much it enhanced the rock fabrics relative to those observed in the core. The present flow behavior implies that a significant improvement was made in permeability, but acidification may simply serve to obscure the real cause of super-k flow. It seems unreasonable to expect that acidification significantly alters reservoir permeability at great distances from the borehole, and so the problem of explaining super-k flow from poor rock fabrics remains. As a viable explanation, acidification probably works only in places where it ‘opens’ up good intervals and connects them with reservoir rock that has better permeability than was measured from the core.
Other explanations proposed for the discrepancy between the petrophysical core measurements and realized flow are poor core recovery from the most permeable part of the flow zone (and therefore no reliable permeability measurements) and the presence of fractures (Moore, 1989). Either would remove the apparent inconsistency between the measured matrix permeability and the measured rate of fluid movement within the super-k interval in wells HWYH-E and HWYH-F. However, incomplete core recovery from intervals correlative with super-k flow occurred in several Hawiyah wells but not in HWYH-E and HWYH-F.
The loss of highly friable and perhaps exceptionally porous core may be inferred in HWYH-D although the loss is minor within the super-k flow interval and cannot be shown on the lithologic log. All core fragments are dolomite and have an exceptionally poor fit around the area of inferred core loss. Furthermore, these poorly fitting core fragments lack fractures and have an average porosity of nearly 30 percent. Quite possibly, the disintegration of intervals that were even more porous than those recovered provides a reasonable interpretation for the missing core.
Poor core recovery, because of fractured reservoir rock in the super-k interval, occurred in HWYH-C and HWYH-G. Ninety-one percent of the oil produced (1,161.8 bf/d/ft) is from a section measuring 15 ft in HWYH-C (Figure 6A). Super-k flow (820 bf/d/ft) is from the bottom six feet, and high flow (342 bf/d/ft) also occurs from the top nine feet. Core plug analysis throughout the 15-foot-thick flow zone indicates that good but not exceptionally high permeability (average 540 md) occurs in the predominantly grainstone interval. Furthermore, the average permeability is higher in the high-flow zone (612 md) than it is in the super-k flow interval (344 md). Fractures readily explain this discrepancy because they override the matrix permeability at the base of the super-k interval. Supporting evidence includes observed fractures in the recovered core, an underlying interval of core rubble within the basal part of the super-k flow interval, and two feet of missing core immediately beneath it. As expected in places where fractures control flow, the super-k interval does not conform closely to the lithofacies boundaries observed in the core. In HWYH-C, the flow extends across the lithofacies boundaries and well beyond the two-foot adjustment limit used to bring about a fit between the lithofacies and the flowmeter readings.
Fractures apparently account for the discrepancy between super-k flow and the low matrix permeability of core from HWYH-G. Water enters the formation through a 10-ft zone at a rate of 712 bf/d/ft and accounts for 55 percent of the water pumped into the field by this injector well. Examination of the core established that a low porosity (average 12.8 percent) and poorly permeable (average 60 md) dolomite occurs in the basal part of this super-k flow interval. It also documented a concentration of fractures within and in the immediate vicinity of the super-k interval. Furthermore, depth-matching the core to logs suggest two missing intervals occur within the interval of fractured core. One of these (four feet) occurs at the top of the super-k zone. The missing core suggests that possible unusual porosity conditions may exist in this part of the well. An examination of the formation density logs and compensated neutron logs precludes the possibility that highly porous and friable dolomite accounts for the missing core because the log porosity decreases slightly over the missing core intervals. Alternatively, the extensive distribution of fractures in recovered core, together with the low porosity-log response over the interval of missing core, offers significant support to the hypothesis that fractures are the most likely cause of the poor core recovery.
The distribution of fractures and missing core intervals shows a strong correlation with the entire interval over which water injection occurs in HWYH-G. This is particularly noteworthy as no correlation exists between the injection interval, lithofacies, or the variation in dolomite porosity-permeability characteristics. Three lithofacies are present within the main zone of injection, but measured porosity and permeability values show only minor variations above, below and within the super-k zone. Consequently, it is concluded that the lack of a relationship between fluid flow and either lithofacies boundaries or variations in permeability distributions is an important indicator of fracture- or fault-controlled fluid flow.
Whereas poor core recovery of the most friable or fractured rocks is consistent with the presence of super-k flow in wells HWYH-C, D, and G, these geologic situations do not explain the super-k in the remaining five wells. Cores from these five wells have pore structures that seems insufficient for sustaining the measured rates of flow within the super-k intervals. Although acid treatment probably made good intervals better than core measurements indicate in three of these wells, artificially enhanced permeability cannot be invoked to explain the super-k flow of the remaining two wells. This incongruous relationship between flow and diminished pore architecture remains unclear.
Invoking explanations of abnormal porosity conditions or fractures near the well bore, but not sampled (Figure 12C), is completely unsatisfactory as the super-k flow must still get to these exceptionally high-flow zones by passing through an ‘ordinary’ pore system. A logical conclusion is that strata with these ordinary pore systems dictate the flow leaving or entering the well bore in a way that is analogous to a filter on the end of a tube. If true, future research into super-k flow should perhaps address questions unrelated to geology. Perhaps variables used in mathematical equations to calculate flow potential based on measured permeability data need to be re-examined. Additionally, relative permeability may hold the key to this particular problem and requires critical examination.
Controls on Stratal Development and Stratiform Super-k Flow Zones
Storm and high-frequency sea-level changes appear to be the main processes that controlled the stratal development of stratiform super-k flow zones. This view is based on two sets of observations. The first is the recognition of tempestite deposits (rhythmic, texturally changing, graded beds that have clearly defined scoured bases) in close relationship with shallow-water lithofacies. The second is ‘non-Waltherian’ stratal relationships wherein marine flooding surfaces sharply separate the deeper-water rocks of one cycle from shallower-water deposits of another (see Cyclicity below).
In Zone 2 of the reservoir (Figure 3), small-scale, upward-fining cycles form beds that vary in thickness from a few inches to about one foot. They generally occur as single beds, but locally the cycles may consist of a stacked succession of two or more beds (Figures 5A, 6A and 9A). Each bed has an erosional base that is overlain by a lower coarse-grained division of graded carbonate sand and an upper division of lime mud; transitions are abrupt between the two. Locally, Thalassinoides burrows (with a backfill of grainstone) riddle the upper muddy division of the stacked beds. The graded beds contain faunal assemblage that are the same as those found in the enclosing shallow-water facies (Hughes, 1996).
The generation of graded beds is a common occurrence in marine environments. The observed sedimentary grading reflects the decay of a heavily sediment-laden current and a declining ability to transport the suspended sedimentary load (Collinson and Thompson, 1982). Results from experimental studies and from fieldwork, have established that such current conditions may develop in response to either storm or turbidite processes (Aigner, 1982; Bagnold, 1954; Edwards et al., 1994; Keen and Slingerland, 1993; Middleton and Hampton, 1973; Nelson, 1982; and Wanless, Terrall, et al., 1988).
In the case of the super-k units, a storm origin seems most plausible as shallow-water facies and fossil assemblages make up the graded beds as well as the general sedimentary sequence of reservoir Zone 2. The observed faunal similarity (Hughes, 1996) is essential in identifying tempestite deposits as they represent sediments that were merely reworked in the same environment of deposition (Einsele and Seilacher, 1991). Turbidite beds differ in that they have mixed faunal assemblages that reflect their shallow-water origin and redeposition in deeper-water environments.
General environmental interpretations for the Arab-D suggest that all tempestites found above reservoir Zone 3 were deposited in shallow water (Hughes, 1996; Meyer and Price, 1993). Graded beds that occur within grain-dominated facies sequences (Figures 4B and 5B) were probably deposited in shoreface settings whereas graded beds from mud-dominated sequences (Figure 11B) formed between normal wave base and storm wave base.
Tempestite deposition commonly produces tabular stratal configurations in shallow subtidal settings (Gagan et al., 1988; Wanless, Tedesco and Tyrell, 1988). Storm stratification is conducive to molding confined flow layers in mud-dominated facies where the layers of mud enclose a highly permeable grainstone. Similar stratification generally does not develop in clean grain-dominated sedimentary environments, as there is no underlying confining mud layer. Nevertheless, exceptions may occur in places where catastrophic storm events deposit tempestites directly onto tight hardgrounds.
Tempestite deposition may also have an expression in dolomite super-k zones. Porous vuggy or moldic dolomite typically alternates with tight mosaic dolomite in stratiform flow layers. In the Arab-D, the mosaic dolomite crystals occur as randomly oriented rhombs with diameters of 200 to 250 microns. Although sparse, replacive anhydrite and anhydrite cements have been seen in association with the dolomite intervals. The alternations of the dolomite textures are spaced so closely that it seems unlikely that they were the result of local variations in the dolomitization process. Instead, the textural differences are most likely to have been inherited, either from the depositional environment or from diagenesis prior to dolomitization. In most cases, the small-scale depositional cyclicity of tempestites parallels the sequences of dolomite. Vuggy or moldic dolomite occurs in what was the graded grainstone division above a scoured surface. It is overlain by mosaic dolomite that corresponds to the originally finer-grained wackestone to mudstone. Excellent examples of preserved scoured surfaces and graded bedding expressed as a progressive upward decrease in the size and density of molds have been observed elsewhere in the Ghawar field (Figure 13). This general parallelism of differences in dolomite texture and in precursor sediments suggests that the composition and/or texture of the tempestite deposits determined the subsequent dolomite texture.
Cantrell et al. (1999) and Cantrell et al. (2000) report that Arab-D dolomites are polygenetic. They invoke four episodes of dolomitization to explain the range of crystal fabrics, isotopes, and trace elements present in the various dolomite layers of the Arab-D reservoir. Only three of the events apply to the dolomite in question. Their results suggest that the formation of massive dolomite in Zone 2 of the reservoir was governed by (1) crystal precipitation from hypersaline brines, (2) early formation of dolomite, and (3) subsequent ‘recrystallization’ of the early dolomite by basinal brines. Corroborating evidence comes from unpublished work by the senior author who proposes that local growth faulting was contemporaneous with Arab-D evaporite deposition and intimately associated with the development of massive dolomite in reservoir Zone 2.
Other authors propose that mixing-zone dolomitization may be responsible for the development of stratiform super-k flow layers in dolomites; for example, Mathews and Frohlich (1998). This dolomitization model receives little support from the geochemical or petrographic data available from Arab-D dolomites and limestones. The most likely candidate for a mixing-zone dolomite is in reservoir Zone 2, but stable isotopes offer little support for such an interpretation. For example, carbon (1.8 to 2.8‰ 13C) is much heavier and oxygen (−3.5 to −5‰ δ18O) relatively more heavy (Cantrell et al., 1999) than the values for dolomite derived from a mixing-zone diagenetic environment (Choquette and Steinen, 1980; Land, 1973; Meyers et al., 1997). Furthermore, the dolomite crystals have strontium contents that range from 100 to 1,000 parts per million (P.K. Swart, University of Miami, written communication, 1999) and local minor developments of replacive or nodular anhydrite. Both the trace element compositions and mineral associations are more consistent with dolomite precipitation from hypersaline brines than from seawater that has been diluted by meteoric water. According to Bray (1997), the diagenetic role of meteoric water as a whole is small.
Leached fabrics are common in pure and dolomitic limestone throughout the Arab-D, but this does not imply the former existence of a meteoric environment. The leaching is primarily fabric-selective after aragonitic grains (Figures 5D and 6C) and is similar to fabric that has known origins in marine waters (Melim et al., 1995; Walter and Burton, 1990). Cement fabrics associated with leached limestones are isopachous fibrous crusts or bladed rinds and syntaxial overgrowths on echinoderms. Rim cements or patchy distributions of blocky calcite, as well as the micro-cavernous porosity zone described by Mathews and Frohlich (1998), are conspicuously absent. The lack of such phreatic signatures below horizons with vadose features (Bray, 1997) is particularly puzzling and brings into question the interpretation of the ‘vadose’ signature and the presence of meteoric fluids in general. Furthermore, the overall style of the Arab-D deposition provides little support for the introduction of meteoric fluids.
With the exception of the uppermost evaporitic cycles in the Arab-D of the Ghawar field, most small-scale depositional cycles have unfilled accommodation space (Figure 3). The most common consist of repetitive lithofacies stacks that record upward shifts into progressively shallower submarine depositional environments. The shoaling-upward stratal units are separated from one another by marine flooding surfaces and are the result of short-term periodic fluctuations in sea level. The forcing takes the form of short-term changes in accommodation space as sea level rises and falls relative to a background of slow tectonic subsidence. Changes in accommodation space effect an orderly and gradational shift in depositional facies. Nowhere, except in anhydrite cycles, does the facies migration result in peritidal deposition as part of the cyclic succession. As no obvious evidence exists for subaerial erosion, it seems unlikely that the absence of peritidal facies is the result of the non-preservation of sediment at the top of submarine cycles. Therefore, the stratigraphic forcing involved relative sea-level oscillations on a small-scale without exposure of the platform. In accepting that sea-level oscillations maintained prolonged periods of submarine deposition, it is unlikely that meteoric fluids could be pumped into the shallow subsurface, especially when there is an extensive evaporitic depositional setting to landward.
Although not supportive of meteoric diagenetic enhancement, the sequential lithofacies variations imposed by relative sea-level oscillations do play a role in the formation of stratiform super-k intervals. Successive orderly and gradational shifts from deposition settings below active wave base to settings above active wave base results in the development of small-scale asymmetrical cycles in which grain-dominated lithofacies become enclosed by mud-dominated lithofacies. Within reservoir Zone 2, these repetitive lithofacies distill into cycles that locally have oolitic, mixed skeletal pelletoidal or fragmented Cladocoropsis grainstones at the top. Representative examples of these cycles define the super-k flow units of wells A, B, and C (Figures 4A, 5A, and 6A) in this study.
Stratigraphic Geometry and Super-k Flow
The absence of flow from, or injection into, stratigraphic intervals whose reservoir rock characteristics are analogous to those of super-k intervals, highlights the importance of stratigraphic geometry and pressure support. Many examples exist throughout the Ghawar field where potential stratiform super-k units do not have significant flow, although the reservoir rock quality of such intervals is better than that found in the super-k zone of the same well. This condition suggests that super-k zones have significantly different stratigraphic configurations and pressure support to facilitate flow in highly permeable confined layers. Many Arab-D producers have had continuous super-k flow for years and even for several decades from intervals less than one foot thick. Thin stratigraphic units must be replenished at high rates in order to sustain this type of reservoir performance. If not, prolonged super-k production would become impossible, as exemplified by production profiles from the fractured Austin Chalk of south Texas. Wells drilled into this reservoir initially produce at high rates from fractures, but production quickly declines over a period of one or more months as the fractures drain. The dramatic production decline occurs because the highly porous, but low-permeability chalk matrix cannot deliver sufficient oil to keep the fracture system filled (Scott, 1977; Stapp, 1977). Similarly, the confluent super-k example described by Moore (1989) demonstrates that oil becomes rapidly depleted from a confined flow layer. Clearly, Arab-D super-k zones with a prolonged production history must be connected to highly porous and permeable reservoir rocks. A consideration of the depositional and diagenetic process discussed above suggests several stratigraphic possibilities. These are (1) tempestite sheets connected to major carbonate shoals, (2) highly prograded grainstone-facies tracts, (3) channel deposits with connections to major carbonate shoals, (4) fault-connected grainstone sheets and banks, and (5) porous and permeable dolomite wedges (Figure 14).
Modern tempestites are known to produce graded sheets in shallow-water carbonate settings (Aigner, 1985; Gagan et al., 1988; High, 1969; Keen and Slingerland, 1993; Wanless, Tedesco and Tyrell, 1988). Thereby, storm deposition limits the position, thickness, geometry, and lateral distribution of the reservoir-seal couplets. Tempestites may be thin grainy sheets (up to two feet thick) isolated within a mud-dominated facies, or the grainy sheet may connect with a major sand body (Figures 14A and 14B). These types of catastrophic deposits could correlate across the entire basin, but they generally have regional applicability only on a scale of 10 to 100 square kilometers. If isolated within a mud-dominated depositional setting, the thin tempestite beds may be charged and pressure-supported by having their basal confining layer perforated (Figure 14A). The perforations are created by Thalassinoides, a burrowing arthropod that typically forms an extensive network of open burrows in the Arab-D firmgrounds. Such burrow networks become super-k ‘feeder pipes’ when they are commonly backfilled with highly permeable grainstone in association with event deposition (S.G. Pemperton, University of Alberta, written communication, 1995).
Alternatively, burrowers are not necessary when storms scour a shallow-water carbonate sandbank and spread it across the platform interior as a thin, graded sheet (Figure 14B). Such a redistribution of sediments is common in modern carbonate depositional environments and, typically, produces tempestite beds that interfinger with the sandbank (Aigner, 1985; Einsele and Seilacher, 1991). The resulting stratigraphic configurations provide the type of connection whereby a large, highly permeable and porous carbonate shoal can deliver sufficient oil to keep the thin tempestite layer filled.
Similar, and perhaps more widespread stratigraphic configurations, may be linked to small-scale oscillations in sea level (Figure 14C). Aggrading shoreface carbonate sand bodies may form thin, regressive, grainstone sheets in response to a base-level fall. Forced to spread out, the high-energy deposits rapidly blanket the mud-dominated offshore facies of the carbonate ramp. Transgressive muddy carbonates will bury the grainstone blanket a short time later as sea level begins to rise. Although this is a possible scenario, the presence of widespread, regressive carbonate sheets is unlikely in the Arab-D as the cyclostratigraphy supports precisely the opposite conclusion. Major differences in cyclic lithofacies composition and the collective arrangement of small-scale cycles confirm that the Arab-D is an overall upward-shallowing deposit that formed in response to marine and coastal facies progradation. Most small-scale submarine cycles have unfilled accommodation space (Figure 3). Grain-dominated lithofacies that form the tops of the cycles lack upper shoreface signatures and show passive surfaces with respect to the transgressive facies of the overlying cycle in middle and outer ramp settings. Thus, small-scale bundling of Arab-D strata into upward-shallowing cycles signals repetitive sedimentological responses to relative sea-level rises followed by stillstands without the intervention of significant sea-level falls.
Although not observed in the present study, channel deposits represent another stratigraphic configuration that promotes super-k elsewhere in the Ghawar field (Figure 14D). These types of shoestring sand bodies are particularly well developed in reservoir Zone 3 where highly porous and permeable molluscan, stromatoporoid and intraclast conglomerates and pelletoidal sands typically make up the channel fill. The channels are cut into and overlain by extensive tight mud sheets that probably represent feeder channels for funneling turbiditic flows into a basinal setting (Meyer and Price, 1993). The channels probably connect in a landward direction with highly porous and permeable carbonate shoals or stromatoporoid buildups, whereas to seaward the channels bifurcate and spread out into lobes with gradational transitions into muddy basinal deposits.
The relationship between faults or fracture systems and super-k flow (Moore, 1989; Valle et al., 1993) is well known, but their supporting role has only been implied in stratiform super-k flow. Thin porous and permeable grainstone layers may be capable of sustained super-k flow because faults connect a series of laterally discontinuous grainstone sheets and prisms (Figure 14E). This configuration probably represents one of the more likely causes of super-k flow, given the widespread recognition of faults in the Ghawar field and the known limited size of individual carbonate shoals down depositional dip (Kerans and Tinker, 1997).
Recent unpublished work by the senior author indicates that growth faulting contemporaneous with the deposition of Arab-D evaporites led to the development of wedge-shaped dolomite bodies (Figure 14F). Mapping suggests that the dolomite bodies developed in response to the downward and subsequent basinward lateral migration of magnesium-enriched hypersaline brines. Low-permeability depositional layers appear to control lateral brine movement (Figure 14F). The resultant dolomite bodies extend downward from the evaporite section, have their thickest development proximal to the inferred faults, and thin and break up into multiple layers in a basinward direction. The dolomite layers preferentially follow mud-dominated carbonate layers. Stacked wedges may form in response to episodic fault movement or the development of additional growth faults that allow the migration of dolomitizing brines. Porosity occlusion and low permeability characterizes that part of the wedge near to faults, whereas incomplete dolomitization typifies the basinward portion of the dolomite wedge. A highly porous and permeable ‘sweet spot’ capable of supporting super-k flow occurs toward the base and center of the dolomite body (Figure 14F). The presence of leached Cladocoropsis (Moore, 1989), and of stromatoporoids, corals or tempestite beds may create the ‘sweet spot’.
A systematic petrographic analysis was conducted on core from eight Arab-D wells that were either producing or injecting at super-k flow rates in the northern Hawiyah area of the Ghawar field. Super-k flow is characterized by stratal arrangements that sandwich a highly permeable unit between two low-permeability layers. This stratiform configuration applies to both limestone and dolomite lithologies. We infer that depositional controls are responsible for the development of stratiform super-k intervals in limestone. In addition, we suspect that super-k flow intervals in dolomite inherited their stratiform characteristics, and that these had a primary depositional original. One possibility is that a stacked succession of dolomitized storm beds (tempestites) deposited within a muddy environment provided the conditions necessary for the formation of stratiform dolomite. Pore systems with touching vugs support some observed super-k flow but, while important to super-k performance, the flow can also come from intervals with an ‘ordinary’ system of interparticle pores. At present, we can only speculate on the geometry of strata that control stratiform super-k performance and on the role of fracture systems in feeding stratiform super-k intervals. However, it is certain that having the right stratigraphic configuration and petrophysical properties does not necessarily lead to super-k flow rates. Pressure support remains a critical component for super-k performance in producing wells. Injectors have this pressure support, and the identification of stratal sequences with the requisite rock fabrics may be used to predict super-k injection rates.
The authors are grateful to Saudi Aramco and the Saudi Arabian Ministry of Petroleum and Mineral Resources for permission to publish this paper. It could not have been written without the cooperation of the entire staff of the Saudi Aramco Core Laboratory. We are very grateful for their prompt response in providing core plugs and laying out core. Special thanks go to Hussain Abusab and Ali Magdad who not only did their usual good job of making the thin-sections, but did so in a very timely manner. We also thank Dr. G. Wyn Hughes and three anonymous reviewers, whose critical reviews helped improve an earlier version of the manuscript. Finally, we wish to acknowledge manuscript improvements by the Geoscience Editor and the graphics design staff of GeoArabia.
ABOUT THE AUTHORS
Franz Meyer has a BS in Geology from State University College of New York, New Paltz and a MS and PhD from the University of Michigan. He joined Shell in 1979, initially as an Exploration Geologist and later became a Carbonate Specialist at Shell’s Belair Research Laboratory in Houston. Franz joined Saudi Aramco in 1991. He is a Geological Specialist engaged in various Jurassic carbonate reservoir and outcrop studies in eastern and central Saudi Arabia. His work includes research into reservoir characterization, sequence stratigraphy, and dolomite sedimentology, as well as teaching.
Rex Price is a Developmental Geologist with Chevron. He has a BS and MS in Geology from the University of Alabama and a PhD from the University of Iowa. After briefly working as a university teacher, an Environmental Geologist with the Alabama Geological Survey, and a kaolin explorationist, he began his petroleum geology career in 1981 at Chevron’s Research Laboratory in La Habra, California. Rex has worked for Chevron in Calgary, in Texas, and on secondment to Saudi Aramco. He has undertaken assignments in research, exploration, geophysics, and development, and as a Carbonate Specialist. He is now working on several Central Basin Platform fields in the Permian Basin of West Texas.
Saleh Al-Raimi has worked as a Reservoir Geologist in the Reservoir Characterization Department of Saudi Aramco since 1981. He has been involved with wellsite operations, hydrology, area exploration, and reservoir geology. Most of his work is now concentrated on modeling reservoirs in Cretaceous, Jurassic, and Permian carbonates.