Reliable, reproducible gamma-ray logs of outcrops have been generated by two techniques which aim to improve the visualization of interwell-scale lateral continuity (and discontinuity) of strata and to demonstrate the reliability and potential pitfalls of subsurface wireline log correlations. One innovative technique was developed which uses a standard gamma-ray sonde run from a logging truck in order to log vertical cliff or quarry faces. A second technique employs a hand-held gamma-ray scintillometer to log more easily accessible outcrops. Examples are presented from the Jackfork Group (Pennsylvanian), Arkansas, USA, of outcrop gamma-ray logging of both laterally continuous and discontinuous turbidites in structurally simple and complex settings. Because the strata from which the logs were measured can be visualized and discussed at the outcrop, these examples can clearly illustrate the following aspects of wireline log correlation: (1) reliability can be greatly improved by understanding expected subsurface depositional geometries and lateral facies changes; (2) wireline log correlations in stratigraphically and structurally complex settings, such as many oil or gas fields, may not be reliable without sufficient coring and special logging in addition to well testing; (3) erroneous correlations can result from the common practice of inferring three-dimensional rock geometries from two-dimensional well log data. Additional examples demonstrate that correlations and interpretations of subsurface wireline logs in both oil/gas fields and in exploration areas can be improved by comparing the subsurface logs with outcrop gamma-ray logs of nearby analogous strata.
The Mt. Messenger Formation exposed along the coastline of the Taranaki Peninsula contains flat-laminated, very-fine to fine-grained sands inlerbedded with bioturbated silt and silty claystone. These beds are typically over 5 cm thick and are the result of high-velocity turbidity currents which allow for good textural sorting and preservation of excellent reservoir-quality sands. Beds less than this thickness, as well as upper parts of many turbidites, typically contain features suggesting reworking, winnowing, and remolding by bottom currents. This winnowing effect is responsible for the good to excellent reservoir properties of the thin, poorly consolidated, well-sorted, very fine-grained lithic arenites. Mt. Messenger sands were deposited on a depositional slope in lower to upper bathyal water depths which was the site for these transforming bottom currents. The Mt. Messenger overlies Mohakatino, Ferry, and Tawariki sandstones and siltstones which were the initial deepwater sediments in the basin. These sandstones are lithic-rich and fine to medium-grained. The beds are typically massiveappearing but locally contain dewatering features, typical ‘classic’ turbidites (graded beds, slurry or low-density turbidity current tops), and were deposited as thick sheet-like bodies on the basin floor. Winnowing of the sandstones did not take place on a large scale and hence reservoir quality is invariably poor to moderate. The mica stilpnomelane in the sands is common; this mineral is restricted areally to low-grade regionally metamorphosed sediments and associated veins, most notably to the schists of western Otago, New Zealand, on the northwestern part of the south island The occurrence of this mineral has important implications as to the source of the Mt. Messenger sediments in the North Taranaki area (transport was from south to north from the South Island).
Hollywood Quarry is a 600 X 375 X 150 ft. (200 X 125 X 50m) excavation which provides a window into lower Pennsylvanian Jackfork Formation turbidite stratal architecture along the crest of a faulted anticlinal fold. A variety of turbidite facies are present, including: (a) lenticular, channelized sandstones, pebbly sandstones, and conglomerates within shale, (b) laterally continuous, interbedded thin sandstones and shales, and (c) thicker, laterally continuous shales. The sandstone and shale layers are broken by several strikc-slip and reverse faults, with vertical displacements of up to several feet. This combination of facies and structural elements has resulted in a highly compartmentalized stratigraphic interval, both horizontally and vertically, along the anticlinal flexure. The quarry can be considered analogous to a scaled-down turbidite reservoir. Outcrop gamma-ray logs, measured sections, a fault map, and cross sections provide a database which is analogous to what would be available for a subsurface reservoir. Thus, the quarry provides an ideal outdoor geologic and engineering ‘workshop’ venue for visualizing the potential complexities of a combination stnictural-stratigraphic (turbidite) reservoir. Since all forms of compartmentalization are readily visible in the quarry, problems related to management of compartmentalized reservoirs can be discussed and analyzed first-hand while standing in the quarry, within this ‘model reservoir’. These problems include: (a) the high degree of stratigraphic and structural complexity that may be encountered, even at close well spacings, (b) uncertainty in well log correlations and log-shape interpretations, (c) variations in volumetric calculations as a function of amount of data available, and (d) potential production problems associated with specific ‘field’ development plans.
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