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LORENZ, JOHN C., Sandia National Laboratories, MS 0750, Albuquerque, NM 87185

ABSTRACT: Natural Fractures in Lenticular Sandstones

Mineralized, vertical extension fractures in the exhumed, fluvial-channel sandstones of the Cretaceous Cedar Mountain Formation (east-central Utah) have strikes that are consistently parallel to the nearby west-northwest trending normal faults regardless of changing orientations in the meandering channels. Fracture intensities (spacings) along the sinuous channels are also only loosely related to channel orientation. Theory, however, would suggest that stresses, and therefore natural fracture patterns such as orientation and intensity, should vary within a sinuous lenticular body enclosed in a shale. These variations should be a function of 1) the differential between the mechanical properties of the sandstone and the shale, and 2) the changing relative orientation between the axis of a sinuous lens and the regional stresses. The discrepancy between observation and theory is In part due to the heterogeneous, sandstone to conglomeratic composition of the Cedar Mountain lenses, which appears to be a more important factor than lens orientation within the stress field. Plots of fracture spacing relative to lens orientation, to grain size, and to mechanical bed thickness, show only irregular trends. A younger set of widely-spaced, unmineralized fractures that trend sub-parallel to the meandering channel axes is also present in the Cedar Mountain sandstones. These fractures were superimposed by gravitational valley-wall stresses and by mobility of the underlying shales during exhumation of the channel axes, and would not be significant to subsurface reservoir plumbing. Knowledge of such fracture patterns would be important to the maximum efficient recovery of hydrocarbons from lenticular reservoirs such as fluvial channels, incised valley fills, and offshore bars.

AAPG Search and Discovery Article #90915©2000 AAPG Rocky Mountain Section, Albuquerque, New Mexico