Characterization of Producing Sands of Oligocene Vicksburg with Inverted Response in Gamma Ray Log in Burgos Basin, Mexico

 

Avalos Torres, Hugo, José Bernal Monjaras, Manuel Morales Leal, Victor M. Alvarez Maya, PEMEX Exploración y Producción, Reynosa, Mexico

 

Burgos basin includes an area of 50,000 Km2, located in the western part of Gulf of Mexico, in the states of Tamaulipas and Nuevo Leon. In the last years has become in one of the most important non associated gas producing basins in Mexico.

The gamma ray displays an anomaly or inverted response in some tertiary sands. This kind of reservoir is located in siliciclastic sediments of Oligocene Vicksburg; and independ­ent between them, besides small size and good permeability (1 to 140 mD).

In more than 60 wells, exploration and development from the fields: Arabe, Cañon, Comitas, Cuitlahuac, Fundador, Kriptón, Pame, Santander and Sigma, the abnormal effect in the gamma ray log is present, and all this reservoirs have gas effects; it looks like shaly bod­ies; this is a really complex behavior, and try to calculate the shale volume with gamma ray and neutron-porosity curves, normally used to obtain it; is impossible; that is why the shale volume is calculated from resistivity and sonic logs like a good alternative.

These reservoirs are gas and condensate producers with rates between 1 and 5.5 mmcfd with pressures from 800 psi to 4000 psi.

The purpose of this work is show how to characterize this kind of reservoirs and to determine the origin in the response of the radioactive logs, based on core analysis and some special logs; using new petrophysical methods knowing pore system, flux units, rock quality index and cementation.

Reservoir Characterization Leading to Record Oil Recovery in a Giant Clastic Reservoir: A Case Study from Lower Burgan in North Kuwait

Azim, Saikh Abdul, Ashok K. Pathak, Tahani Al-Saig, Shehab A.M. Abdullah, Kuwait Oil Company, Ahmadi, Kuwait

The Lower Burgan is a giant clastic reservoir under active production for last 45 years. The reservoir accounts for 74% of the total production over North Kuwait. Even after achiev­ing a recovery factor of 44%, it is still on its plateau production with 10% water cut. An aggressive multidisciplinary effort is being pursued to achieve a record recovery of 65% under primary recovery. Integration of different vintage of static and dynamic data leading to an improved reservoir model capable of explaining the reservoir heterogeneity is the key factor.

The Lower part of the reservoir deposited as coalesced braided river channels is under strong bottom water drive and most part of the reservoir is swept by rising bottom water. As bulk of the remaining oil is in the Upper part deposited as estuarine channel complex and shore face sheet deposits, it is the focus of a detailed reservoir description. Refined layer­ing integrating core sedimentology, sequence stratigraphy and layer-wise pressures defined precise flow units. Individual channels have been mapped from an integrated paleocurrent analysis. Infill locations are being drilled to target bypassed oil along these channels. Geological factors leading to the complex pattern of water encroachment have been identi­fied in different parts of the field and the finding is a key input to prolong low watercut. Elemental Capture Spectroscopy log has been used to characterize porosity accurately using variable matrix density. The thin sands and net/gross are being evaluated with multicompo­nent 3D resistivity tools. Characterizing residual oil saturation and defining facies-based rel­ative permeability are critical in achieving the ultimate recovery.

The paper describes the characterization process leading to this record high recovery in detail.