The aim of this study is to propose a stratigraphic and sedimentary framework though the integration of available sedimentary, diagenetic and petrophysical data, which will be utilized in the construction of a high resolution stratigraphic framework, as an input into comprehensive review and update of an existing model of heterogeneous carbonate reservoir in a mature field in Abu Dhabi, UAE.
Depositional facies have been defined in cored wells, subsequently were associated taking into account the biologic and sedimentary processes in response of carbonate growing and sea level changes, allowing the identification of the main stratigraphic surfaces.
Surfaces can extend the correlation along the field and define the model of facies that, with the evidence provided by cores, can recreate and predict the different regressive-transgressive cycles in high resolution which the carbonate platform were undergone during its evolution.
Diagenetic evolution, interpreted through laboratory observations, was integrated with facies and petrophysical evaluation allowing the understanding of the spatial distribution of petrophysical properties within a heterogeneous reservoir and define a new set of facies which will be used in the generation of geological static model.
Application of sequence stratigraphy methods in cores, and extended in logs allowed the identification of six depositional sequences, with thicknesses of 2 to 4 meters each, corresponding to the phases of carbonate platform growth. Within each depositional sequences, typical cycles were defined that support the understanding in the association of facies and their relationship during the deposition.
The identification of sedimentological cycles not only genetically organizes the facies and predicts the stacking pattern, but also makes possible to find an excellent correspondence between cycles from lowstand system track intervals with good to excellent permeability values, and cycles from transgressive system track intervals with low permeabilities.
Many of the sequence stratigraphy published articles driven for the most important reservoirs along the Arabian Plate, provide an excellent tool in the regional correlation. However, they are not enough to be used in the reservoir characterization in detail that is required during the development of the field neither as input data in the generation of geological static models that use the sedimentary trends as constrain to populate the petrophysical properties.
The carbonate reservoirs lithofacies discussed in this paper contain heterogeneous pore types and properties. The challenge in predicting the distribution of the pores properties is through the technique used to construct a representative model to effectively describe the lithofacies distribution in 3D.
The studied reservoirs are part of the Lekhwair and Kharaib Formations that were deposited in carbonate platform environment varied from lagoon, shoal, to ramp settings during Valanginian-Barremian age. Core and well-log data from an onshore oil field of Abu Dhabi were used to establish lithofacies distribution schemes and high resolution sequence stratigraphy (HRSS) frameworks. Twenty four lithofacies were identified based on faunal content, texture, sedimentary structures, and lithologic composition. HRSS interpretation indicated twenty one fourth-order parasequences that displayed aggradational, progradational, and retrogradational stacking cycles.
A novel stochastic multirealizations modeling of lithofacies and their property distribution has been developed to characterize the subsurface complexity. The method combined depositional environment, HRSS, and diagenesis trend, and then integrated them with dynamic data to generate a holistic reservoirs characterization and representative simulation model.
The workflow is as follows: Detailed core, thin section and lithofacies description; Paleoenvironmental interpretation; HRSS interpretation; Field scale stochastic modeling; Flow simulation and history match validation.
Detailed core, thin section and lithofacies description;
Field scale stochastic modeling;
Flow simulation and history match validation.
Field wide reservoirs lithofacies and property distributions were modeled and constrained by the identified HRSS framework. Hybrid combination of Truncated Gaussian Simulation (TGS) and Sequential Indicator Simulation (SIS) algorithms were used. This allows generating equiprobable multirealizations of realistic lithofacies cycles and properties trend in the area. The use of lithofacies distribution provided flexibility in the modeling workflow as it offers the outline for rock properties distribution such as rock types, porosity, permeability, and water saturation. The resulting multirealizations demonstrated consistency with the conceptual scheme, outcrop analogs, and geostatistical trend.
This innovative approach has recently been implemented successfully in the studied field reservoirs. The resulting dynamic model depicted a good production history match; hence it will provide reliable production forecast and reservoirs development plan.
Due to a limited number and poor quality of the available core samples, sandstone analogues are required for a research project on sand production in Kazakhstan. A consistent sample preparation method is essential to achieve repeatable test results in terms of the deformational characteristics and the failure strength of the rock. This paper presents results of the work conducted to produce synthetic sandstone samples by firing clay and sand mixture. The samples replicate the reservoir sandstones in terms of the particle-size distribution, cement content, mineralogy and porosity. The reservoir sand was used as a material for the sandstone samples. The samples with 20% and 30% clay content were prepared. Effect of temperature and firing duration was studied by firing the samples at 200°C for 5 hours, 200°C for 8 hours and 500°C for 5 hours. Scanning Electron Microscopy analysis was conducted on the samples to study the chemical transformation during firing and its effect on clay cementation. Triaxial test behavior of the reservoir sand and sandstone samples was investigated.
Sand production is one of the main problems associated with weak reservoir sandstones at shallow depths in Kazakhstan oilfields. The reservoir sandstones of the oilfield from the Ustuyrt-Buzachi Sedimentary Basin are confined to the Lower Cretaceous formations of Hauterivian and Barremian ages. The reservoir depth varies from 200 m to 500 m. The paleoclimate was warm and humid during Hauterivian period, sedimentation occurred at deltaic depositional environment, followed by sea transgression during Barremian period and deposition of shallow-marine sediments (Tashliyev and Tovbina, 1992). As a result of such climatic and depositional conditions, the reservoir is composed of fine-grained sandstone with weak clay cement (Worden and Morad, 2003). Clay content of the reservoir rock varies from less than 10% up to 35%, reservoir properties diminishing with higher clay contents.
Due to a limited number and poor quality of the available core samples, sandstone analogues are required for the experiments. A consistent sample preparation method is essential to achieve repeatable test results in terms of the deformational characteristics and the failure strength of the rock. Different synthetic sandstone preparation methods have been developed and reported in the literature. Usage of Portland cement and gypsum as a bonding material has been widely practiced (Coop and Atkinson, 1993, Nouri et al., 2004, Consoli et al., 2012). Another series of techniques can be grouped under the method employing silicates to bond the sand particles. Cementation by silica glass was achieved by Bernabe et al., 1992 by mixing Ottawa sand with very fine powder of pyrex borosilicate glass, compacting at 14 MPa and heating to 950°C. Holt et al., 1993 mixed sand with sodium silicate solution, and flushed the mixture with CO2 under 'in-situ' stress. Mixture of fine Fontainnebleau sand and alkaline silica gel was exposed to high pressure and temperature by Broke et al., 1997. Calcite In-situ Precipitation System (CIPS) is employed in several works (Ismail et al., 2000, Sherlock and Siggins, 2003), where calcite solution is injected into the sand sample, and as a result of chemical processes within the fluid crystalline calcite precipitates, coating the sand grains and forming calcite bridges at particle contacts. Calcite precipitation can also be initiated and mediated by biological processes. Several studies (Rong et al., 2012) made use of alkalophilic microbe and calcite ion solution to achieve particle bonding, the technique is known as Microbial Induced Calcium Carbonate precipitation (MICP).
The Upper Kharaib Member of the Lower Barremian in Abu Dhabi onshore is represented by the Formation-B, which is separated from the overlying Shuaiba formation by 45 to 50 feet of dense Limestone of the Hawar formation. It is classified into B-Upper and B-Lower, comprising seven reservoir quality sub-zones (BI, BII, BIIIU, BIIIL, BIV, BV and BVI) separated by six stylolitic intervals. The average thickness for B-Lower is about 100 ft as compared to 60 ft in the upper zone. This study provides integrated sedimentological and diagenetical characteristics - in a field-scale - of the Formation-B in Abu Dhabi, UAE.
A unified lithofacies scheme has been used to describe the Formation-B. These litho-facies have been grouped into eight genetically related lithofacies associations that reflect their corresponding deposition environments. The stacking of the lithofacies associations (inner ramp, mid-ramp and mid to outer ramp) define the broad 3rd order trends observed across the study field within the Formation-B, which have been compared to the regional sequence stratigraphic framework of
The lateral lithological variations occur at a higher order (i.e. 4th/5th order), most likely driven by autocyclic topographic/hydrodynamic variations, in addition to sea level changes; these will impart lateral reservoir heterogeneity. Within subzones BIV to BI, variations in lithofacies associations are observed due to the patchy nature of the higher energy inner ramp facies.
The study concluded that the main controllers on reservoir quality distribution are texture and primary composition, as the matrix and grain ratio, together with allochems type, abundance, and size, define the genetically distinct characteristics of the lithofacies associations and provide the precursor fabric for subsequent diagenetic processes to occur. Cementation affects all the lithofacies; the morphology and pervasiveness of the crystals are dependent on primary and secondary pore space available for cementation. The key pore enhancing phases are dissolution events, mainly early and late dissolution enhancing micropores and enhancing/creating macropores, commonly partially or completely negating the effect of cementing phases. As the effects of the diagenetic overprint are linked to primary texture and composition, the distribution of the lithofacies and lithofacies associations will influence reservoir quality. The grainier lithofacies are more commonly seen to be influenced by pore-enhancing late dissolution phases, thus these facies host better reservoir quality. This is best developed in the upper portion of the Formation-B. The combined understanding of the sedimentological framework and the diagenetic overprint provides a robust tool for predicting the reservoir architecture.
When water is needed for water injection in oil fields of an arid country, far from sea, the water supply is facing various technical and economical problems. The only solution is to find underground problems. The only solution is to find underground waters unattractive for any other possible use. In the selection of water sources, oil exploration is a big chance for underground aquifer surveys and knowledge. This paper shows how it has been used in defining the best way to provide the oil industry water needs and in evaluating possible interferences with other uses such as regional agricultural development plans. plans. First geological studies from oil exploration or development wells allowed to reach a good definition of aquifer extensions and thicknesses. Then water well tests first led to a good knowledge of hydro-geological characteristics. Finally, mathematical models were specially built to define the aquifer deliverabilities, the best water well schemes and the regional consequences on water levels.
Combined use of the two deep aquifers, confined turonian aquifer at an average of 850 meters below ground level with very salty water and artesian albian-barremian aquifer at an average depth of 1400 meters below ground level with fresh water, has been shown necessary to satisfy oil injection water needs and local industrial and domestrical water consumption that together will reach about 1 m /s during the next ten years.
Oil from the giant Hassi Messaoud field is extracted from the Cambrian formation since 1956. The field is divided in several parts, called "zones". Many of these zones are passing below the bubble point and will require pressure maintenance as soon as the critical gas saturation will be reached. In two pilot areas, miscible flood has been replaced since 1973 by water injection ; this water is supplied from some water wells drilled in the turonian aquifer. But reservoir engineering studies have defined new water requirements for these two zones and some ten others. The strong increase in water needs asked so for a complete and precise study of water supply capabilities of the different existing aquifers. Among the four main water-bearing formations, which are from bottom to the surface, jurassic, albian-barremian, turonian and miopliocene, the study concentrated on the two intermediate deep aquifers as the jurassic formation, tested by oil exploration wells, is not productive and as the miopliocene (water table) productive and as the miopliocene (water table) aquifer, which has a very high deliverability, contains fresh water to be preserved for human consumption and agricultural development.
DESCRIPTION OF WATER BEARING FORMATIONS
Oil exploration, with wildcats scattered in all the northern Sahara sedimentary basin, proved the continuity of geological formations which are water-bearing. Two fundamental works, Busson's geological study and water management ERESS project from UNESCO , have gathered an huge amount of geological and hydrogeological data. General description of the aquifer formations is drawn from their works. Figure 1 gives a rough description of northern Saharian basin, from Morocco to Tunisia; it shows albian-barremian outcrops which are either recharge or leakage areas, the main streamlines and the two great ergs covered by sand dunes. Figure 2 shows the cross-section from Reggane to Matmata (troglodyte village) passing through Hassi Messaoud field; it presents the different water bearing formations and presents the different water bearing formations and gives a rough idea of their depths and thicknesses. Piezometry for albian-barremian aquifer and Piezometry for albian-barremian aquifer and miopliocene-eocene indicates that the first one is artesian and the second one a water table. Jurassic and turonian aquifers, with salty waters, are confined.
Section I/D, Paper 6 STRATIGRAPHIC CORRELATION OF UPPER CRETACEOUS AND LOWER CRETACEOUS IN THE TETHYS AND BOREAL BY THE AID OF MICROFOSSILS BY F. BETTENSTAEDT AND C. A. WICHER (*) SYNOPSIS. Stratigraphically most important are the following microfossils: the foraminifera in the marine complex; the ostracods in the brackish and fresh water complex and the spores in the terrestrial complex. Due to ihe indenting of these facies, on the basis of these microfossils, it will be possible in the future to establish a world-wide correlation comprising all facies. This time only a small part is being dealt with, viz., some marine small foraminifera of the Cretaceous. In the course of intensive drilting for petroleum a reliable micropaleontological subdivision of the Cretaceous controlled by megafossils was established in northern Germany. Samples from other countries proved that many index fossils do also occur btyond Germany in almost all European countries, in Israel, Egypt, Morocco, U. S. A., Mexico and Trinidad. Of greatest importance is the ecological difference between cretaceous Sediments of the Tethys and the Boreal. The species occurring in both facies show the same vertical range and sequence almost everywhere. In order to use foraminifera for universal correlations their stratigraphic distribution should, with the help of ac- curately horizonted samples, be fixed and thesame species should everywhere be given th,e samespeciflc name. There are three examples chosen out of many others io demonstrate the obvious likenesses. The reasons for the confusion in taxonomy and nomenclature of foraminifera are to be seen in the neglect of the extensive literature, in inadequate figures, chiefly, however, in the insufli;cient knowledge of the width of variation. According to observations of the writers, minor difierences of the width of variation of the individual species permitting the establishing of subspecies (geographic races) only are found when comparing faunas of the same geological age from far distant localities. By that, world-wide fossils may everywhere retain the same specific name, a factor that facilitates stratigraphic comparison and simplifies complicated systems (ternuer nomenclature). On two stratigraphic tables a certain number of index fossils of the marine small foraminifera are presented. They are species with world-wide distributim occurring simultaneously in the Boreal and the Tethys, or such species discovered in the Boreal only or in the Tethys only, occurring, however, universally within these facies. Special importance is put on benthonic foraminifera for reason of stressing their usefulness for the establishing of world-wide corre- lations. Short explanations are given for the 55 foraminifera species (31 in the Upper Cretaceous and 24 in the Lower Cretaceous) as to their stratigraphic and geogra,phic range. 24 species are figured on three fossil-p