Until recently, an exploration petrophysicist expecting a pay zone with good porosity and high resistivity might simply have disregarded a conglomerate reservoir on the Norwegian continental shelf. The conglomerates observed in the North and Barents seas are mineralogically complex and present either low-resistivity/low-contrast or low-porosity/low-hydrocarbon pore volume conditions. However, in recent years, newer measurements and evaluation techniques have become available, which in addition to conventional logs, have been used to enhance the petrophysical evaluation of a number of important oil and gas conglomerate discoveries made in both siliciclastic and carbonate settings in the North and Barents seas. These Jurassic-Triassic aged reservoirs hold economically viable contingent reserves and exhibit production rates varying from 1,000 BOPD to more than 3000 BOPD.
Conventional logs acquired in the carbonate conglomerates of the Barents Sea and the conglomeratic sandstones of the North Sea have proven difficult to interpret. Low porosities, varied mineral distributions, heterogeneous pore systems, low resistivity contrasts between hydrocarbon- and water-bearing intervals, variation in rock texture, and the presence of immovable hydrocarbons can represent a formidable petrophysical evaluation challenge. Even at depths where the presence of hydrocarbon is established, formation testing in some cases results in either tight (low-mobility) tests or flows water during sampling. For this reason, hydrocarbon moved by mud filtrate invasion is often a better indicator of producible pay than that inferred from favorable hydrocarbon saturations alone.
We present an integrated petrophysical evaluation technique combining induced gamma ray spectroscopy measurements, used to create a reliable lithology/porosity model, with dielectric dispersion measurements, used to provide fluid saturations. The resulting analysis accurately reveals the subtle differences in shallow versus deep saturations that are critical in predicting movable hydrocarbon in the conglomerates. Provided as a timely delivery prior to formation testing, the predictive power of the petrophysical evaluation is illustrated by agreement with the subsequent formation testing data.
Four field examples, one from the North Sea and three from the Barents Sea conglomerate reservoirs, are discussed in this paper. The proposed evaluation technique is based on an integrated petrophysical analysis of dielectric dispersion, induced gamma ray spectroscopy, and standard log measurements. The method has consistently proven successful at defining intervals containing producible pay in multiple wells across the complex and varied conglomerate discoveries on the Norwegian continental shelf.
