This paper discusses the results of an innovative methodology using Dionisos 4D forward stratigraphic modelling of Middle Eastern carbonate reservoirs on a field scale. Traditional stochastic techniques do not sufficiently capture carbonate reservoir heterogeneities, reducing the accuracy of static and dynamic models. The methodology applied to three Lower Cretaceous UAE reservoirs, uses a deterministic approach that aims to define carbonate heterogeneity and provides a structure to develop a more accurate and usable static and dynamic model for field development. Dionisos uses a predefined sequence stratigraphic scheme as a framework. A reference case model is manually calibrated to environmental parameters, followed by automated multi-realisations that generate several other plausible calibrated models. A sensitivity analysis provides an indication of the influencing environmental parameters controlling facies and texture distribution.The calibrated forward stratigraphic models resulted in the generation of 14 carbonate textures for the three reservoirs using a 200x200 m grid size and a 50 kyrs time step. Carbonate lithology production (mud, fine, coarse, bioconstructions) varies between 0 and 350 m/Ma, wave direction is SW (200 -260°); wave action depth 7-18 m while wave energies vary between 0 and 140 kW/m. Sediment diffusion coefficients by wave transport range from 0.1 (mud) to 0.0008 km 2 /kyr (bioconstructions) while gravity driven transports from 0.1 to 0.001 km 2 /kyr. The lower part of Reservoir A is characterised by low angle TST sequences dominated by algal boundstones-floatstones. Deposition continued with the development of a low relief margin with aggradational to progradational architectures comprising rudist shoals. This defined a topographic split into platform, slope and basin with lateral texture heterogeneities showing a northward deepening trend. The successive clinoform top sets (Seq4a, b) are rich in rudist boundstones-floatstones with lower slope dominated by packstones-wackestones.Reservoir B and C are isopach with strong lateral variability in carbonate texture as evidenced by well data. The overall architecture of the sedimentary systems consists of low relief interconnected algal boundstone-floatstone mounds separated by gentle depressions dominated by fine grained sedimentation. The numerical simulation of these systems was driven by a carbonate production law as a function of the substratum energy and bathymetry under dynamic subsidence/uplift conditions. The innovative workflow applied at field scale allowed the modelling of complex carbonate geometries and associated textures honouring lateral and vertical heterogeneities observed at wells. The application of this workflow as alternative/complement to stochastic methodologies brings further insights on the proposed sequence stratigraphic framework allowing the confidence and predictability of static and dynamic facies models to be increased.
Concentrations of organic carbon and selected major and trace elements are presented stratigraphically for a 100-m cored section of Cenomanian through Santonian calcareous black shales from the Maracaibo Basin, western Venezuela. Significant geochemical variation within these laminated, organic-rich marls suggests a five-part stratigraphic subdivision for the La Luna Formation in the Alpuf-6 (ALP-6) core recovered by Maraven, South America. Geochemical variation is interpreted to reflect the combined influences of relative sea level change, episodic volcanism, variations in the intensity of upwelling, and subtle geochemical alteration. High concentrations of organic carbon and trace metals in La Luna strata are inferred to result from (1) moderately high accumulation rates of organic matter resulting from pulsed high primary productivity, (2) recurrent episodes of widespread oxygen depletion that enhanced accumulation of both organic carbon and trace metals in localized areas, and (3) reduced terrigenous dilution of marine organic carbon and trace metals caused by high sea level and low rates of bulk sedimentation in pelagic settings.A refined biostratigraphic framework for the ALP-6 core allows calculation of accumulation rates for early-middle Turonian, late Turonian, Coniacian, and Santonian time intervals. Calculated accumulation rates of organic carbon and trace metals are highest during the late Turonian, when bulk sedimentation is highest. Concentrations of organic carbon are highest during the Coniacian, when bulk sedimentation rates are lowest. Calculated organic carbon accumulation rates for the La Luna in ALP-6 range from approximately 100 to 400 mg/cm 3 /k.y., similar to rates estimated for mid-Cretaceous strata from a number of other sites but approximately an order of magnitude lower than rates measured beneath modern upwelling zones. Relative to organic carbon and bulk sediment accumulation rates, the calculated accumulation rates of selected metals in La Luna strata are unusually high when compared with rates of metal accumulation in many modern Davis, C., Pratt, L.
The field, a Late Ordovician gas condensate reservoir located in the Illizi Basin of the Algerian Sahara Desert, was put on production 6 years ago. It is the off-centred gas cap of a greater oil reservoir whose production was initiated by Sonatrach in 1967. The reservoir is characterised by its extreme geological complexity. The Late Ordovician (Unit IV) corresponds to a glacial period in the Saharian Basins. The heterogeneity of the reservoir -especially the strong variations in thickness -is such that it is extremely difficult to predict the performance of new wells, even when located in a well developed area. The presence of fractures organised in corridors increases even further the reservoir complexity. This paper presents the ways in which those challenges have been tackled in order to obtain both a reliable geological and dynamic model. It is shown how this characterisation was made possible by the integration of all the available information: seismic, geological and dynamic data.The matrix modelling was performed though the definition of electrofacies. Outcrop observations, depositional models as well as seismic data were used to define the internal layering of the geological model.The fracture model was achieved by integrating the curvature of the top reservoir horizon. It was confirmed and refined through comparison with borehole imagery (BHI) logs and dynamic data (mainly well tests and historical production data). Fracture conductivity was adjusted such that well productivity was respected. The outcome was a Discrete Fracture Network (DFN) model that was then upscaled and integrated into a single medium dynamic model.The final outcome is a dynamic model in which the history matched process was greatly improved. The predictability of the model was tested in an under-developed area of the field where new wells have recently been drilled. A fairly good agreement between the model and the reality was found in areas already appraised. In completely undeveloped area, the model was less successful.
During this innovative work a 3D Stratigraphic forward Modeler initially designed for geohistorical basin modelling has been used to produce multi-realization of 3D facies distribution of a giant offshore carbonate reservoir in Abu-Dhabi. The Stratigraphic 3D forward Modeler simulates carbonate production and transport by solving a diffusion equation. The modeling started from a given age, through a sequence time steps. At each time step, three main parameters controlling deposits are modelled: 1) Accommodation space which reflecting the total available water depth for sediment deposits, 2) Sediment Supply by setting carbonates production laws inside the model, 3) Transport and Wave Reworking using a diffusion law, function of wave energy, slope and a diffusion coefficient depending on carbonate nature and grain size. The model is calibrated to facies thickness and texture described at 17 cored wells by a previous 4th order Sequence Stratigraphic Sedimentology study. The model has a grid of 200 X 200m, similar to the reservoir simulation grid size, a 50,000 years time step which corresponds to 136 layers, ensuring a vertical resolution in the range of 1 to 2 meters. The total size of the model is 2.7 Million cells. The resulting model reproduces the spatial facies distribution in the reservoir and with more than 80% success the facies texture and thickness at cored wells for each 4th order sequences. By varying the parameters controlling sediment deposits, using an Experimental Design approach, several realizations of the geological model have been produced to capture uncertainties on facies distribution between wells, each of them honoring the facies description and thickness at cored wells. This new approach increases considerably the consistency, resolution and reliability of the geological model between wells.
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