Recognition of By-Passed Oil Zones in Cretaceous Clastic Reservoirs: Case Study From Caricare Field, Northern Llanos Basin, Colombia

With the discovery of Caricare field in 2006, in the northern Llanos Basin of Colombia, one of the main productive reservoirs has been a Cretaceous interval named the K2A sub-unit. The depositional environment of the reservoir is interpreted to be shallow marine, and two predominant deposits have been identified: shoreface deposits and tidal channel deposits. In the southwest part of the field, the productive zones of the reservoir are usually identified as having log resistivity values ranging from 20 ohm-m to 30 ohm-m. However, the same interval in the central and north parts of the field, have lower log resistivities, around 5 ohm-m to 14 ohm-m, which equate to high-calculated water saturations. To get a better understanding of the variation in the resistivity response, an integrated formation evaluation study was completed, which combined well logs, core analysis data, electrofacies analysis, petrophysical interpretation and reservoir engineering data. As a result, the zones of low resistivity were recognized as oil-bearing. These zones, previously considered to be non-pay, were reclassified as pay. Subsequent testing of the low resistivity zones resulted in initial production of oil with moderate water cuts. This case study shows how resistivity log responses are probably affected by a combination of fine grain size, bioturbation, and clay content. Careful, integrated petrophysical analysis of these zones, coupled with an understanding of the causes of low resistivity, have resulted in the recovery of reserves that would otherwise be left behind pipe.