Summary. Massive, stratigraphically discordant dolomite occurs on the late Mesozoic Arabian Shelf in the northern portion of Aramco's main producing area. The dolomite is associated with solution-collapse of anhydrite seals and with enhancement of porosity and permeability in tight limestone seals within the region. By destroying regional caprocks. dolomitization has had an adverse effect on oil accumulation. The spatial distribution of this regional dolomite was mapped with wireline log and core data. Geochemical and fluid-inclusion analyses indicate that the dolomite formed from hot saline brines that were first expelled from halite-bearing evaporites, and then migrated into Arabian Shelf carbonates during burial. Introduction Some of the most prolific petroleum reservoirs in the world occur in Upper Jurassic and Lower Cretaceous carbonate formations in Saudi Arabia. Most reservoirs are composed of pelletal, oolitic, or bioclastic shoal grainstones that have high primary porosity and permeability. These reservoirs are sealed either by tight limestone permeability. These reservoirs are sealed either by tight limestone or by massive anhydrite (Fig. 1). The seals are effective throughout most of eastern Saudi Arabia. Around the rim of the Gotnia salt basin at the northern edge of Aramco's main producing area, however, regional stratigraphically discordant dolomite (i.e., dolomite that cross-cuts formational boundaries) has enhanced porosity and permeability in tight carbonates that seal reservoirs in the porosity and permeability in tight carbonates that seal reservoirs in the Tuwaiq Mountain, Hanifa, Jubaila, and Sulaiy formations and has rendered them ineffective (Fig. 2). The dolomite is also associated with solution collapse and destruction of anhydrite seals in the Hith and Arab formations. This study was undertaken to determine the origin and distribution of the dolomite so that dolomitized areas could be avoided when drilling programs are planned. Dolomitization Processes. Limestone (CaCO3) is converted to dolomite [CaMg(CO3)2] in a number of different environments, 1.2 usually through a solution/reprecipitation process. Dolomitization by surface brines in supratidal evaporite flats (sabkhas) in the Holocene has been well documented and is often used to explain the origin of ancient dolomites that are associated with evaporites. Mixing seawater with meteoric water may produce a favorable chemical environment for dolomitization in some instances, and is commonly used to explain ancient dolomites not associated with evaporites. Few occurrences of mixing-zone dolomite have been documented in Quaternary sediments and rocks, however, so debate continues over the importance of this process. Dolomitization by seawater is thermodynamically favored but does not occur on a large scale because of kinetic limitations. Recently, however, examples of dolomitization by seawater have been reported. Finally, increasing evidence suggests that a significant percentage of ancient dolomites may have formed in the subsurface through the interaction of limestones with interstitial brines during burial. Dolomitization can enhance reservoir quality by increasing permeability and, in some cases, by increasing porosity. Because permeability and, in some cases, by increasing porosity. Because dolomite is less reactive than calcite, dolomites are also more resistant to porosity loss with depth than limestones. Therefore, the spatial distribution of dolomitized intervals within a carbonate section often defines the limits of reservoir development. The hydrologic process that dolomitizes a limestone can control the morphology process that dolomitizes a limestone can control the morphology of the dolomite body as well (e.g., sabkha dolomitization can produce thin. stratified reservoirs. while subsurface reef-front produce thin. stratified reservoirs. while subsurface reef-front dolomitization can produce thick, "shoestring" reservoirs. Therefore, if we wish to predict the spatial distribution of a dolomite body. it is advisable first to determine the process that produced the dolomite. produced the dolomite. With this in mind, an integrated petrographic, geochemical, and subsurface mapping project was designed to delineate the distribution of Arabian Shelf dolomites and to determine their origin. Core, Lithology Log, and Petrographic Studies Cores and lithology logs of the Tuwaiq Mountain, Hanifa, Jubaila, Arab, Hith, and Sulaiy formations from 28 Arabian Shelf wells were examined and the occurrences of host dolomite (dolomitized limestone). baroque dolomite (white sparry dolomite that fills fractures and vugs). and accessory minerals were tabulated. Representative wells from 11 fields were examined in thin-sections. These samples were later subjected to stable-isotope and fluid-inclusion analysis. Dolomitized intervals have a very distinctive mineral assemblage, composed of the following. Host Dolomite (or Dolomitized Limestone). Dolomitization usually obliterates textures of precursor limestone. However, where relict textures are preserved, they indicate that host dolomite formed by the alteration of open marine grainstones and packstones, rather than in a restricted, evaporitic environment. This suggests that host dolomite did not form at the surface in an arid tidal-flat/sabkha setting. Baroque Dolomite. This white, sparry secondary dolomite fills vugs and fractures and is strongly associated with collapse breccias. Although baroque dolomite most commonly forms as a direct Precipitate-into open pores. it sometimes shows a replacement rela Precipitate-into open pores. it sometimes shows a replacement rela tionship with host dolomite. Generally. it is impossible to determine the paragenetic relationship between host and baroque dolomite using thin-section petrography alone. Accessory Minerals. These include, pyrite, sphalerite, fluorite, and coarsely crystalline anhydrite. Thin-section examination indicates that all these minerals precipitated more or less contemporaneously with baroque dolomite and with each other. Maps and Cross Sections Percentage dolomite was calculated for each of the six formations Percentage dolomite was calculated for each of the six formations in 50 wells from compensated formation density (FDC) and compensated neutron logs (CNL) by the method discussed here. Formation tops and bottoms were picked in each well. The number of feet of evaporites was subtracted from total formation thickness to give the number of feet of net carbonate in each formation. SPEFE P. 435

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The Khuff Formation can be described as a second-order transgressive-regressive sequence.The transgressive sequence set comprises Khuff third-order composite sequences KS7 through KS5 and the third-order lowstand/transgressive systems tract of KS4. The regressive (highstand) sequence set comprises the highstand systems tract of KS4 as well as Khuff third-order composite sequences KS3 through KS1. Major Khuff gas reservoirs are associated with the second-order highstand sequence set.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractSome of the key challenges facing producing organizations are to increase recovery from existing fields and to optimize the utilization and value of existing infrastructure. The Middle East is uniquely positioned to help supply world energy demand through efficient application of secondary and tertiary recovery techniques in super-giant carbonate reservoirs. Three long-life carbonate fields in the USA are offered as examples of the benefits achieved by a continuous process of data collection, studies, and systematic application of available technologies. This continuous process has progressively increased ultimate recovery. The example fields will achieve a range of incremental increases in the recovery factor of between 8 and 20 percent original oil-in-place (% OOIP) or up to 50% OOIP increase over conventional primary recovery. A systematic and integrated approach to reservoir management has been employed to understand the basic rock and fluid physics of each reservoir and the key parameters that impact performance. The development plan for each of the example fields has then been implemented in a way that maximizes both hydrocarbon recovery and value of the assets while utilizing best available technologies.

Stylolites can be an important geologic feature affecting reservoir quality and, consequently, reservoir management, in many carbonate reservoirs. Thin, discontinuous cemented zones associated with stylolites occur in the massive, high porosity dolomites of the Upper Smackover at Jay/LEC Field and are the source of horizontal baffles to vertical flow suspected since early days of production and corroborated by full-field reservoir performance studies. It was the need for thin, vertical flow baffles to match historical waterflood arrivals in both full-field and small-area simulation modeling that led to the extensive re-examination of the core and recognition of these previously undetected cemented zones associated with stylolites. Field-wide conventional coring provides a superb core database for describing the physical nature and distribution of stylolites and associated cements. The Smackover interval is cored in over 90% of the wells (149 of the 163 wells). Porosity, permeability and fluid saturations were measured on one-inch core plugs sampled at one-foot intervals. However, small scale heterogeneities such as the reduced permeability associated with the cemented zones above and/or below stylolites were usually not captured using this unbiased core sampling procedure. The cemented zones vary from a few millimeters to several centimeters thick. Probe permeameter analysis has been used to document the decreased permeability adjacent to the stylolites. Three-dimensional geostatistical models of porosity, horizontal matrix permeability and the distribution of stylolites and cemented zones were constructed and used to derive the reservoir properties required for mechanistic simulation models. Reservoir simulations were run on models with and without the cemented zones to (1) determine what impact the cemented zones have on field performance and (2) examine alternative operating strategies for the current miscible nitrogen flood that was initiated in 1981. In the simulations, the cemented zones were assumed to have no effect on porosities and horizontal permeabilities. However, new scaled-up values of vertical permeability were generated to reflect the distribution of cements in the geologic stylolite/cement model. Results show that within the Smackover at Jay/LEC Field, the stylolite-induced baffles enhance oil recovery by reducing gravity segregation and improving the sweep efficiency of the injected nitrogen. Introduction The Upper Jurassic (Oxfordian) Smackover Formation is one of the most prolific hydrocarbon-producing formations in the Gulf Coast region. The producing trend extends in an arcuate pattern around the northern rim of the Gulf of Mexico basin from Texas to Florida. Jay Field, the largest of the Smackover fields, is located in Escambia and Santa Rosa counties, Florida near the eastern border of the Florida Panhandle (Fig. 1). It extends northward into Escambia County, Alabama where it is referred to as Little Escambia Creek (LEC) Field. Jay/LEC Field is approximately 7 miles long and 3 miles wide (Fig. 2). The Smackover Formation is at depths of 15,000 feet (4500 meters) to 16,000 feet (4800 meters) and has an average thickness of about 350 feet (105 meters). Oil is trapped in a northeast-trending anticline on the downthrown side of the Foshee fault. The fault forms the eastern barrier to oil migration and an updip trap to the north is formed by a facies change from porous dolomites to tight limestones and evaporites. Jay/LEC Field was discovered in June 1970 by the Humble St. Regis Paper Co. #1 wildcat drilled six miles south of known production. Jay/LEC Field was unitized and a waterflood using a 3:1 line drive pattern was initiated in 1974 to arrest the rapid pressure decline observed during primary depletion. The current miscible nitrogen flood, based on the water-alternating-gas (WAG) process, was initiated in 1981 to increase reserves and extend the field life. Cumulative oil production from Jay/LEC Field is in excess of 415 million barrels. P. 213^