Devonian Naturally Fractured Reservoirs

Abstract

Abstract

This paper evaluates the naturally fractured reservoirs (NFR) that produce most of the gas in southern Bolivia and northwest Argentina in order to find tools and models that could lead the exploration efforts to new discoveries. These reservoirs of the Tarija basin are known as Huamampampa and Santa Rosa formations dated as early Devonian.

A reasonable amount of public information related to geosciences and engineering, published in both countries, has been reviewed and evaluated. Field trips have also been done to describe the outcrops in the vicinity of the producing fields, observe the fracture patterns, and measured their frequency and orientation. Electric logs, wellbore images, core analysis, pressure and production data from some key wells have been obtained, analyzed, and integrated into this study.

Gas production comes from tight sands broken in two main set of fractures, oriented N10oW and N80oE, as well as several others that convert the reservoir into polyhedrical pieces which resemble a box of matches. The open fractures communicate vertically sandstone and shale layers in such a way that they constitute a single reservoir.

Several geological factors have made these tight sands the main reservoirs in the Subandean belt: thick layers with some primary porosity and abnormal high pressure, interbeded or encapsulated in a moderately rich source rock. Total Organic Content from wells drilled in the studied area varies from 0.4% to 1.9 % in the Silurian Kirusillas as well as the Devonian Icla and Los Monos formations which have fed the reservoirs. Fractures and abnormal pressure explain the reservoir connectivity and high initial production rates, ranging from 30 to 200 MMscf/day per well. Total combined ultimate recovery of six fields adds up to 14 TCFs, 55% of the original gas in place.

Modern seismic on this high relief mountainous terrain has been proven effective to image synclines and limbs of these narrow faulted structures, but must be improved to map better the fold axial plane of the deep target reservoir where most of the fracturing has occurred. Integrating seismic, surface geology, fracture orientation, logs and core data into the model is required to reduce the risk of 5,000+ meter deep wells. Investments in large 3D seismic sound too high, but will be paid off since infrastructure and natural gas markets are in place.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016