Prediction of macrofracture properties using microfracture information, Mesaverde Group sandstones, San Juan basin, New Mexico

Predictive Modelling of Naturally Fractured Reservoirs Using Geomechanics and Flow Simulation

San Juan basin is a roughly circular, asymmetric structural depression located in northwestern New Mexico and southwestern Colorado. Ultimate recoverable reserves of predominantly nonassociated gas (23 tcf, 0.65 × 1012 m3) are present in the structurally low part of the central basin. The major producing intervals are low-permeability sandstone reservoirs in the Upper Cretaceous Dakota Sandstone, Mesaverde Group, and Pictured Cliffs Sandstone. Lesser amounts of oil and/or gas are produced from Pennsylvanian, Jurassic, and Cretaceous rocks along the southern and western flanks of the basin. The gases display a trend of becoming isotopically heavier (^dgr13C1 values range from -48.7 to -31.4^pmil and chemically drier (C1/C1-5 values range from 0.75 to 0.99) with increasing depth. These changes are assumed to be the result of thermal cracking processes, and the gases are interpreted to have been generated during the mature and post-mature stages of hydrocarbon generation. However, there is considerable scatter in the data which is interpreted to result from a difference in source rock type. Gases generated from nonmarine (humic) source rocks are isotopically heavier and chemically drier than those generated from marine (sapropelic) source rocks at equivalent levels of maturity. The gases also become isotopically heavier and chemically drier to the northeast, following the trend of increasing maturity of all units in that direction. The increase in maturity is attributed to a combination of greater burial depth and a higher geothermal gradient resulting from batholiths to the north in the San Juan Mountains area. Maximum burial and heat flow occurred during the Oligocene, which probably coincided with peak hydrocarbon generation. Lack of oil in the central basin is believed to be the result of two factors. First, gas in reservoirs such as the Dakota Sandstone may have resulted from thermal cracking of oil generated from marine source rocks during late mature (wet gas-condensate) and post-mature (dry gas) stages of hydrocarbon generation. Second, gas in reservoirs such as Mesaverde Group and Pictured Cliffs Sandstone is nonassociated and probably was generated from nonmarine (coaly) organic matter during the mature and post-mature stages. Minor amounts of condensate, instead of oil, may have been generated from nonmarine source rocks during the mature stage.

Relation of Natural Gas Composition to Thermal Maturity and Source Rock Type in San Juan Basin, Northwestern New Mexico and Southwestern Colorado

Abstract The mechanics of deformation in multilayer flexures is analysed by comparing field observations of joint clusters from the East Kaibab Monocline, Utah with fracture patterns produced in analytical and numerical experiments. Dune boundaries (bedding planes) were mapped through the thickness of the aeolian Navajo Formation, and the occurrence of joints related to dune boundary slip and fold curvature was documented. Slip along dune boundaries, as evidenced by joint clusters oblique to bedding, occurs along the steep limb of the fold and in the middle of the Navajo. Joints perpendicular to bedding and parallel to the fold axis occur near the synclinal hinge. Numerical experiments examine a layer flexed to match the Navajo at Hackberry Canyon with both uniform and observed distribution of dune boundaries. Within the numerical experiments, horizontal frictional interfaces slip within the centre of the layers where dips are steepest, and opening-mode fractures related to curvature form within the anticlinal and synclinal hinges of the fold. Thus, the first-order numerical results match field observations. This study illustrates the important roles of mechanical stratigraphy and interlayer slip in multilayered folding and the contribution of bedding-plane faults and fold curvature in the production of joint clusters.

Interlayer slip and joint localization in the East Kaibab Monocline, Utah: field evidence and results from numerical modelling

Fissure formation and subsurface subsidence in a coalbed fire

Fracture reservoirs are present in the Verde, Boulder, West Puerto Chiquito, and East Puerto Chiquito oil fields of the San Juan basin, New Mexico. Brittle, competent siltstone and carbonate-rich interbeds within Cretaceous shale intervals are fractured in areas of maximum curvature along the Hogback monocline. Surface observations indicate that there are usually three sets of fractures along limbs of folds. Open fractures trend parallel with fold axes and occur on convex sides of folds. Dip fractures and oblique fractures are commonly tight. Exploration for traps with fracture reservoirs should focus on areas of maximum curvature of folds where brittle interbeds can be expected in the subsurface. Monoclines are common along the margins of many basins in the Rocky Mountain region and provide many exploration targets, as they probably formed at relatively low confining pressures, thus facilitating fracture of brittle rocks in the Cretaceous System.

J. F. Callender

Papers

Fractures in Cretaceous Rocks from Selected Areas of San Juan Basin, New Mexico--Exploration Implications