3D Geological Modeling in a Turbidite System With Complex Stratigraphic-Structural Framework-An Example From Campos Basin, Brazil

Abstract

Abstract After 3 years of production, results indicated necessity to refine the geological model of Cretaceous turbidite reservoirs in an off-shore oil field of Campos Basin, Brazil. A stratigraphic analysis was developed to build a new stratigraphic-structural framework. Seismic interpretation was used to incorporate structural data by mapping the system of normal faults. Using concepts of sequence stratigraphy, seismic, biostratigraphic and lithologic data were used to define 12 major depositional sequences, spanning from Cenomanian to Maastrichtian. In 8 of these sequences, 12 turbidite systems were recognized and 9 of them focused for the present study. These turbidite systems compose the operational reservoir zones. Six of them were deposited during the Turonian and three during the Santonian. Erosive surfaces and regional unconformities affect the distribution of the turbidite systems. Action of erosion and faulting results in complex framework and lateral communications among reservoirs of different ages. Several oil-water contacts are present and controlled by some of the faults and the lateral communications. Modeling this complex system included many different techniques and softwares in steps of the work. The first step involved a topologic three-dimensional construction, including all of the major geologic information; relationships among the main turbidite systems were validated by production data. Next was creating a refined grid to populate the model with rock-properties; corner point geometry grid was built to honor the direction of major faults. Third was facies characterization; vertical proportion matrix was used to represent the horizontal non-stationary of the data and seismic amplitude was used as a constraint. Plurigaussian simulations were used to populate the model; this type of simulation is suitable to represent the multiple and non-sequential contact relations among facies. The fifth step was using Monte Carlo simulation to fulfill the model with porosity and permeability, based on petrophysic histograms from well data, classified by facies. Upscaling the model was the last step in order to finally transfer the geological data to the flow simulator. Preliminary results from the flow simulator reveal better fitting between this new geological model and the present production data. Introduction Along the years from 2000 to 2002, a series of perforations and workovers in this studied field indicated a partial inadequacy of the model then existent to explain the deviations found in relation to the expected results. A multidisciplinary team (petrophysicist, geophysicist, sedimentologist, reservoir geologist and engineer) was composed to perform an integrated study to update the geological model. An extensive revision work of all the available data and their internal organization into a three-dimensional (3D) model, to respect the main complexities and heterogeneities observed in the field reservoirs, was then put in practice. Part of the methods and results is next exposed. Preliminary Information The studied field is composed of 6 different blocks distributed in an area of 728 km2 with water depths varying between 800 and 1500m. The present work deals with its main producing block. The main reservoirs are Upper Cretaceous (Turonian and Santonian) turbidite sandstones, with good quality oil (28° API). A large number of stacking and partially connected reservoirs is recognized in different turbidite systems. These turbidites are characterized as amalgamated channel complexes. An increasing erosive character upwards causes, especially in Santonian reservoirs, erosive superimposed surfaces, including some unconformities. These complex stratigraphic sequences were later disturbed by normal faults, during episodic salt movements. Main flow restrictions should be related (i) to structural subjects (complex fault system) (ii) external geometry and internal heterogeneity of the reservoirs and (iii) the structural relationship among the different stratigraphic units.