Reservoir-Scale Controls on Fracture Orientation: Structural Position Versus Mechanical Variation

Abstract

Characterizing fracture networks across a single field has been a consideration within the petroleum industry for decades but has become increasingly more important with the exploitation of unconventional plays such as tight sandstone and carbonate fields. Recent work has considered the importance of the mechanical stratigraphy in fracture network formation, and whether the present-day mechanical stratigraphy is an accurate predictor of the fracture stratigraphy or fracture network. A third question concerns the scale over which mechanical stratigraphy can influence the generation of fractures. This study considers fracture orientations in several positions across a single anticline, together with the mechanical and diagenetic history of the stratigraphic sequence. Measurements were taken on the crest of the anticline and on both the gently-dipping backlimb and the more steeply-dipping forelimb. At each location, bedding and fracture orientations, and lithology were recorded and unit hardness was measured with a rebound hammer. Samples were collected for thin section analysis of the diagenetic history. Results for the anticline limbs indicate that there are two characteristic patterns, one for each limb, which are not influenced by variation in hardness between individual beds. On the crest of the anticline, a third characteristic fracture pattern can be identified, with some variation between the patterns developed in dolomitic mudstone and the overlying tight sandstone. Again, individual beds within each lithology show variations in hardness with no change in fracture pattern. Thin-section analysis indicates late-stage (i.e. post fracturing) diagenetic changes to the units which are expected to influence the present-day hardness. This data suggests that in a highly deformed area such as a fold-thrust belt, the structural position is the strongest control on the developing fracture pattern, followed by large-scale variations in lithology and diagenesis rather than bed-scale variations in hardness. In less deformed areas, smaller-scale mechanical variation may have a greater influence. In addition, those diagenetic processes occurring after major fracture development will also affect which fracture orientations can be used as fluid-flow pathways.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015