RECOGNITION AND CAUSES OF LOW RESISTIVITY PAY ZONES IN CRETACEOUS CLASTIC RESERVOIRS: A CASE STUDY FROM AN OILFIELD IN NORTHERN LLANOS BASIN, COLOMBIA.

摘要

Over the years, low-resistivity pay has become recognized as a worldwide phenomenon, and is present in Cretaceous clastic reservoirs of Caricare Field, Northern Llanos Basin of Colombia. The causes of low resistivity can be explained through integration of geological, petrophysical and reservoir engineering data. Understanding the causes of low-resistivity pay would result in the recovery of reserves that would otherwise be left behind pipe. The Caricare field is currently being produced from one of the Cretaceous sands, and has been since 2006. The reservoir has a strong edge water drive. Produced oil is 33.5° API with low GOR. The reservoir sands are quartz- rich, generally medium-grained, with fine-to-medium-grained interbeds. The dominant clay is kaolinite which appears as crystallized booklets, generally filling pore spaces and replacing grains. Average porosity is 24-30%, with permeability ranging from 500mD to 2000 mD. The reservoir has a homogenous capillary pressure profile throughout the interval. The depositional environment of the Caricare field is interpreted to be shallow marine. Two predominant marine facies have been recognized: a shoreface facies, containing coarsening upward sequences in the southwest part of the field; and a tidal channel facies, containing blocky to fining upward sequences, in the central to the north parts of the field. Oil productive zones in the southwest of Caricare are usually indentified as having resistivity values ranging from 20-30 ohm-m. However, the same intervals in the central and north of Caricare have lower resistivities, around 5-14 ohm-m, which calculates high water saturations. In numerous fields around the world, clays formed during and after deposition is the primary cause of low-resistivity pay. They can be distributed in the formation as laminar shales, dispersed clays or structural clays. The effects of shales on the resistivity, spontaneous potential (SP), porosity, gamma ray (GR) logs, permeability and water saturation calculations are well known. This case study shows how resistivity log responses are affected by a combination of fine grain size, bioturbation and clays, and when applied in a detailed petrophysical analysis, results in better recognition of pay intervals with low resistivity. The core samples from CC-5 well showed oil saturations in zones previously calculated as non-pay intervals. The zone was tested and initially produced oil with moderate water cuts. Scanning electron microscope (SEM) and X-ray diffraction (XRD) results identified several mechanisms contributing to high conductivity: Clay minerals (chlorite, illite and kaolinite) Fine-grained sandstones and feldspars Bioturbation which created small vertical clay tubes in clean sands. Shale laminations Quartz was the most prominent mineral identified, followed by kaolinite. Porosity is classified as primary intergranular up to 24 %; pore size ranged from 20u to 350u.