Abstracts, Carbonates & Evaporites, v. 14, no. 2, 1999, December

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PETROGRAPHY AND ORIGIN OF DOLOMITE IN CARBONATE DEPOSITS IN VARIOUS PALEOCLIMATES

Vitaly G. Kuznetsov
The State Academy of Oil and Gas, Leninskiy pr., 65, 117917 Moscow, Russia

ABSTRACT: Dolomite distribution in carbonate formations of various ages and climatic zones has been studied. Vendian-Cambrian carbonate deposits of the Siberian Platform and Tithonian (Upper Jurassic) carbonate sediments of the North Pre-Caucasus were formed in arid environments. Tournaisian (Lower Carboniferous) deposits of the eastern Russian Platform were formed in a humid climate.

In arid formations primary deposition of magnesium occurred in peritidal- and sabkha facies. In ancient sequences, the dolomite origin was primarily chemogenic, biochemogenic, and biogenic and the textures formed were microgranular. In younger sequences the dolomite had an early diagenetic origin and magnesium deposition was chemogenic and biochemogenic; its texture is fine-medium crystalline. In humid formations primary deposition of magnesium in shallow waters is not significant. Comparatively high quantities of magnesium were deposited in relatively deep marine sediments in association with organic matter. Substantial quantities of dolomite were found only in reefs, the dolomitization being late diagenetic, metasomatism, the texture is medium crystalline to macrocrystalline.

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GEOLOGIC ASPECT AND ESR DATING OF CORALS FROM PLEISTOCENE TERRACES IN RAS MOHAMMED, EGYPT

H. Ochiai, M. Ikeya, and *M.A. Morsy
Department of Earth and Space Sciences, Osaka University, Toyonaka, Japan
*Nucl. Mat. Authority, Cairo, Egypt

ABSTRACT: Extensive Pleistocene coral reef terraces are developed along the coasts of the Gulf of Aqaba and Suez as well as the Red Sea. Three terraces show five reef stages. ESR dating of corals from marine terraces at Ras Mohammed, Sinai, Egypt has been made to study the sea-level changes. The ESR ages of corals at the Terrace I (0-5m), Terrace II (5-10m), and Terrace III (10-20 m) are 150 ka, 220 ka, and 140 ka, respectively. Some samples are calcified and hence give ages that are younger than the true ages.

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FACIES DIFFERENTIATION AND SEQUENCE STRATIGRAPHY IN ANCIENT EVAPORITE BASINS - AN EXAMPLE FROM THE BASAL ZECHSTEIN (UPPER PERMIAN OF GERMANY)

Ingo Steinhoff¹,* and Christian Strohmenger²,**
¹Department of Geological Sciences, The University of Texas at El Paso, El Paso, Texas 79968
*BP Amoco, 501 West Lake Park Boulevard, Houston, Texas 77079, USA
²BEB Erdgas und Erdoel GmbH, Riethorst 12, D-30659 Hannover, Germany
**Exxon Exploration Company, 233 Benmar, Houston, Texas 77060, USA

ABSTRACT: Due to excellent preservation, the Werra Anhydrite (A1), the upper member of the Upper Permian Zechstein cycle 1 (1st cycle, Z1), is readily studied in terms of the distribution of sulfate facies and sequence stratigraphy that can be interpreted from these facies. In this study cores taken from seven wells in the Southern Zechstein Basin were examined for their sedimentary structures and various petrographic features. Facies interpretation and depositional sequences are based on detailed examination of core material.

Four main facies environments have been identified: (I) supratidal, (II) intertidal, (III) shallow subtidal, and (IV) deeper (hypersaline) subtidal. These are further subdivided into 10 subfacies types: (1) karst and (2) sabkha within the supratidal environment (I), (3) algal tidal-flat, (4) tidal flat, and (5) beach deposit within the intertidal environment (II), (6) salina, and (7) sulfate arenites within the shallow subtidal environment (III). The (8) slope subfacies type commonly associated with (9) turbidites and the (10) basin subfacies type subdivide the deeper subtidal environment (IV).

Vertical stacking patterns of these facies and subfacies types reveal the sequence stratigraphic development of the sulfate cycles in response to sea-level and salinity fluctuations.

The lower Werra Anhydrite (belonging to Zechstein Sequence ZS2) is characterized by a transgressive systems tract (TST) overlying the transgressive surface of Zechstein Sequence ZS2 within the A1-underlying upper Zechstein Limestone (Ca1). The TST of the A1 is several tens of meters thick in platform areas, where it is built up by sulfate arenites and swallow-tail anhydrite-after-gypsum, and thins out to a few meters of thickness toward the condensed basinal section, where laminites ("Linien-Anhydrit") are predominant.

Most of the A1 succession consists of three relatively thick parasequences belonging to the highstand systems tract (HST) that shows typical prograding sets. Enhanced platform buildup, including sulfate arenites, salina deposits, intertidal sediments, and sabkha precipitation, as well as turbidite shedding off the platforms produced marginal "sulfate walls" up to 400 m thick as platform to slope portions of the Werra Anhydrite. Seaward, the A1 thins to a few tens of meters of laminated sulfate basin muds. Increasingly pronounced A1 topography during highstand narrowed the slope subfacies belt parallel to the platform margin. This contrasts with the broad but considerably thinner slope deposits of transgressive times with much shallower slopes.

The ensuing sea-level lowstand is reflected by a sequence boundary on top of the karstified A1-platform and a lowstand wedge (Zechstein Sequence ZS3) overlying portions of the slope and basinal subfacies of the A1 highstand systems tract. Beyond the lateral limits of the lowstand wedge, the sequence boundary merges with the transgressive surface of ZS3, shown by the lithologic change from the A1 anhydrites to the overlying carbonates of the Stassfurt Carbonates ("Haupt Dolomit" Main Dolomite, Ca2).

The Basal Anhydrite (A2), which overlies and seals the carbonate reservoir of the Ca2, can also be subdivided into systems tracts by means of facies analysis. It is, however, much less complex than the A1 and is comprised almost exclusively of a transgressive systems tract of Zechstein Sequence ZS4.

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GEOCHEMISTRY OF WATERS IN THE CANALOBRE CAVE AND AQUIFER OF CABEÇÓ D'OR (SOUTH-EAST SPAIN)

¹J.M. Andreu, ²J.C. Cerón, ³A. Pulido-Bosch, and ¹A. Estévez
¹Department of Earth Sciences and Environment, University of Alicante, 03080 Alicante, Spain; Phone: 34 96.590.34.00-3341; Fax: 34 96.590.35.52; [email protected]
²Department of Geology, University of Huelva, 21819 Palos de la Frontera, Huelva, Spain; Phone: 34 959.35.05.99; Fax: 34 959.53.01.75; [email protected]
³Department of Hydrogeology, University of Almería, 01420 La Cañada, Almería, Spain; Phone and Fax: 34 950.21.54.65; [email protected]

ABSTRACT: The Canalobre Cave, situated in the Sierra del Cabeçó d'Or in the south-east of Spain is a shallow-hole of approximately 100 m depth of cylindrical morphology. Rainfall events of a certain magnitude cause precipitation in the interior of the cave. Taking advantage of these events, a series of samples was taken with the aim of characterizing the hydrochemistry of the rapid infiltration waters within this zone of the unsaturated zone, and defining the processes which modify their chemical composition. At the same time, an attempt was made to understand their relationship with the waters of the saturated zone of the Cabeçó d'Or aquifer. Use of principal components analysis (PCA) and a study of hydrodynamic equilibria established a primary contribution from the edapho-climatic component, which enriches the infiltration waters with salts and CO₂ and so favors dissolution of the rock during the downward passage of the water. In the saturated zone, where there is a remarkable thermal anomaly, dissolution of evaporite salts at depth, together with the possible contribution of CO₂ of endogenous origin (associated with gasification-degasification phenomena that cause dissolution-precipitation processes), is the likely explanation for the considerable increase in the mineralized status of the waters, and for the correlation between its content of various ions.

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MINERALOGY AND WATER CHEMISTRY OF THE LAKE ACIGÖL, DENIZLI, TURKEY

Halim Mutlu, *Selahattin Kadir, and *Aydo an Akbulut
For correspondence: Osmangazi University, Department of Geological Engineering, Bademlik 26030 Eski ehir, Turkey; [email protected]
*General Directorate of Mineral Research and Exploration of Turkey (MTA) 06520 Ankara, Turkey

ABSTRACT: Lake Acigöl of Turkey is a perennial lake with a brine composition of Na-Cl-SO₄. The lake is fed mainly by two distinct water sources. The first is groundwater of Mg-HCO₃ type and springs of Na-SO₄ type. The difference in composition of inflow waters is attributed to different ways of water cycling. Ephemeral mudflats around Lake Acigöl are composed of gypsum, calcite, dolomite, huntite, together with clastic minerals. Brine-soaked mudflat exist locally and contain efflorescence of halite, bloedite, thenardite and/or mirabilite precipitating from the artesian groundwater. Dominance of aragonite and high TOC values recorded in the gel-like lake sediments are the indicator of bacterial activity in the lake. Fluid-mineral equilibria calculations performed on the waters of the Lake Acigöl basin successfully predicted precipitation of minerals detected in the recent sediments.

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PALEOKARSTS IN LATE PRECAMBRIAN AND ORDOVICIAN CARBONATES, KALPIN-SHAYA UPLIFT ZONE, TARIM BASIN, CHINA

^1,2Hongsheng Cao, ²Jiduan Yang, and ²Daning Wang
¹Department of Geology, Florida State University, Tallahassee, FL 32306-4100
²Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China

ABSTRACT: The reservoir properties in the Kalpin-Shaya uplift zone, Tarim basin, are a common concern with regards to petroleum exploration and reservoir evaluation alike. Dissolution and paleokarst have a positive impact on the porosity as well as the storage capacity of carbonate reservoirs because the secondary porosity related to dissolution and paleokarst serves as excellent traps for migrating hydrocarbons. In order to evaluate the reservoir characteristics reasonably in the late Precambrian and Ordovician carbonate rocks, the secondary porosity, which was produced by dissolution and paleokarstification in the late diagenetic stage, should be studied because the primary pores were mostly destroyed during the early-middle diagenesis due to serious compaction and multi-cementation.

Carbonate rocks are among the most important collectors of oil and gas accumulations in the world. Important oil and gas reservoirs in paleokarst-containing carbonate rocks are known worldwide because micropores and megapores, such as solution openings, solution fissures, funnels, sinkholes, and caves, serve as the fundamentally important secondary porosity in those rocks. Several wells revealed that the Kalpin-Shaya region is a prospective target for oil and gas exploration.

The reservoir carbonates of the Kalpin-Shaya uplift zone in the northern Tarim include dolomites and limestones. The best dolomite reservoirs are in the late Precambrian Qigebulake Formation (Z₂²), the lower Qiulitage Group (E_2-3), the upper Qiulitage Group (O₁¹), and the Xiaoerbulake Formation (E₁), whereas limestone reservoirs are in the middle-upper formations of the upper Qiulitage Group (O₁^2-3).

On the basis of the study of petrology, paleontology, and stratigraphy from field work and well core data, the pore spaces within the Precambrian and Ordovician carbonate reservoirs are studied with the aim of proving that all secondary pores are controlled by dissolution and paleokarst.

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PETROLOGY, MINERALOGY AND SEDIMENTATION OF THE COASTAL QUATERNARY CARBONATES IN QATAR, ARABIAN GULF

¹Mohamed M. Abu-Zeid, ²Abd El Monem T. Abd El Hameed, and ³Alia J. Al Kuwari
¹Department of Geology, Faculty of Science, United Arab Emirates University, P.O. Box 17551, Al Ain, UAE
²Department of Geology, Faculty of Science, Tanta University, Tanta, Egypt
³Department of Geology, Faculty of Science, Qatar University, Doha, Qatar

ABSTRACT: Twelve stratigraphic sections representing the Quaternary carbonate sequence along the coastline of Qatar were the subject of field and laboratory investigations. The latter comprised grain-size analyses, semi-quantitative identification of the acid-insoluble residues, microscopic examination of thin sections and X-ray diffractometry.

The studied sequence is composed mainly of calcarenites. It comprises three lithostratigraphic units having markedly different structural, textural and compositional characteristics. These units are variably recorded along the coastline of Qatar and may show remarkable lateral variations in thickness and facies characteristics.

Sediments of the oldest Unit I were deposited in shore-zone and shallow marine environments during a transgressive phase of the Pleistocene sea. Their major components were transported to the sea by several river systems directed mainly to the southeast and north and drained the Upper Miocene-Pliocene Hofuf Formation and the Eocene Rus and Dammam Formations. Regression of the sea resulted in the development of terrestrial aeolian conditions under which sediments of Unit II accumulated mainly in the form of dunes. These sediments were inherited primarily from older marine deposits especially those of Unit I. The sequence of Unit III represents a second transgressive phase which was less profound than the first phase. Fluviatile processes were more active. Various river systems, directed primarily towards the east, drained the mainly Eocene rocks.

The predominantly arid conditions which prevailed in Qatar during the Pleistocene were intermittently interrupted by semi-arid and humid climates, especially during the Late Pleistocene.

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