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Reservoir Geology of the Carmopolis Oil Field, Brazil

1985; Canadian Society of Petroleum Geologists; Volume: 33; Issue: 4 Linguagem: Inglês

10.35767/gscpgbull.33.4.379

2368-0261

A. Candido, N. C. Wardlaw,

Geological and Geophysical Studies

ABSTRACT The Lower Cretaceous Carmopolis Member within the Carmopolis oil field is composed of syntectonic conglomerates and associated finer clastic sediments deposited in an alluvial-fan complex within fault-bounded grabens. Four major lithofacies are distinguished on the basis of grain size, which decreases progressively to the east in the direction of sediment movement. Grain density decreases as grain size becomes smaller, due mainly to decreasing content of garnet and pyrite. Garnet decreases in the direction of sediment movement related to depositional factors whereas pyrite formed postdepositionally. The coarser-grained lithofacies have higher bulk density because of higher average grain density as well as lower average porosity. These relationships facilitate recognition of the major lithofacies from density logs and gamma ray logs where core is not available. Porosity-permeability relationships were established from core for each major lithofacies. At constant porosity, permeability increases with increase in grain size but the coarser-grained facies have lower average porosity and permeability. In wells lacking core, major lithofacies and porosity determined from bulk-density logs are used to estimate permeability indirectly, using core-derived porosity-permeability relationships. Decreasing sediment heterogeneity in the direction of transport was caused by greater uniformity of fluid flow over the depositional surface and lack of sediment transported directly by gravity forces. Well log and core data indicate higher oil-to-water ratios in the coarser-grained lithofacies. However, porosity, permeability and pore structure become more variable as grain size increases. Thus, reservoir quality in the coarser-grained lithofacies is poorer because of their lower porosity and permeability as well as their greater heterogeneity. LA GEOLOGIE DE LA ROCHE-RESERVOIR DU CHAMP PETROLIFERE CARMOPOLIS, BRESIL RESUME Le membre Carmopolis du Cretace inferieur, partie integrante du champ petrolifere Carmopolis, est compose de conglomerats syntectoniques et de leurs sediments clastiques moins grenus, dans un complexe de cones de dejection encaisse par les failles de grabens. On distingue quatre lithofacies principaux, bases sur la granulometrie qui decroit progressivement vers l'est, dans la direction du transport des sediments. La densite des fragments detritiques diminue a mesure que le grain devient plus fin, surtout a cause des quantites decroissantes de grenat et de pyrite. La quantite de grenat diminue dans la direction du transport des sediments et est reliee a des facteurs de sedimentation, tandis que la pyrite est formee apres la sedimentation. Les lithofacies a grain plus grossier ont une densite apparente plus importante a cause de la densite moyenne plus elevee des fragments detritiques, et de la porosite moyenne moindre. Ces rapports facilitent l'identification des principaux lithofacies a partir de diagrammes de la densite et de diagrammes du rayonnement gamma, lorsque des carottes ne sont pas disponibles. Des rapports ont ete etablis entre la porosite et la permeabilite, a partir de carottes, pour chacun des principaux lithofacies. A porosite constante, la permeabilite augmente avec la granulometrie, mais les facies a grain grossier ont en moyenne une porosite et une permeabilite moins elevees. Pour les forages sans carottes, les lithofacies principaux et la porosite sont determines a partir de diagrammes de la densite, et servent ensuite a estimer la permeabilite indirectement en faisant usage des rapports etablis entre la porosite et la permeabilite, provenant de l'analyse de carottes. L'heterogeneite des sediments decroit dans la direction du transport a cause de la plus grande uniformite de l'ecoulement du fluide sur la surface de sedimentation, et de la penurie de sediments transportes directement par les forces de gravitation. Les donnees provenant de diagraphies et de l'analyse de carottes indiquent une proportion huile-eau plus elevee dans les lithofacies plus grenus. Par contre, la variabilite de la porosite, de la permeabilite et de la structure des pores augmente avec la granulometrie. Donc, les lithofacies plus grenus constituent des reservoirs de moindre qualite, a cause de leur porosite et de leur permeabilite moins importantes, et pour des raisons de plus grande heterogeneite. Traduit par Marc Charest