Regional Caprock-Destroying Dolomite on the Middle Jurassic to Early Cretaceous Arabian Shelf

Abstract

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