Fracture Architecture and Mechanical Stratigraphy in the Monterey Formation and its Relationship to Sedimentary Cycles at Montaña de Oro, California
Strickland, Heather; Behl, Richard J.; and Gross, Michael
The Monterey-equivalent Miguelito Member of the Pismo Formation at Montana de Oro State Park displays four orders of fracture length that can be related to stratigraphic position in primary sedimentary cycles of bed-thickness and composition. Characterization of fractures such as these is crucial to understanding reservoir behavior in low-permeability, fine-grained rocks that require natural or induced fractures for economic hydrocarbon production. We define the fracture network and mechanical stratigraphy – the subdivision of a rock section into discrete units defined by mechanical layer boundaries – at scales from cm’s to 10’s of m.
A ~200-m-thick interval of the upper Miocene Miguelito Member consists principally of rhythmically interbedded porcelanite, mudstone and dolostone; sedimentary cycles in porcelanite:mudstone ratio, bedding thickness and dolostone occurrence are quantitatively defined by spectral gamma ray and ground-based three-dimensional LiDAR (light detection and ranging). We hypothesize that primary sedimentary cyclicity influences subsequent fracture and fault development (and the resulting mechanical stratigraphy) in the Miguelito Member and likely other thin-bedded siliceous successions. and therefore may be used as a predictive tool for fracture frequency and length in conjunction with other geologic information such as tectonic strain and structural position. We are mapping the dimensions of fractures and faults and calculating their frequencies in relation to the overall stratal stacking pattern and variations due to sedimentary cyclicity. As fracture type and frequency are known to be directly related to lithology in the Monterey Formation on a single-bed scale, intervals where strata are thinner-bedded and have a higher silica:mudstone ratio are predicted to develop a higher number of both bed-confined fractures and multilayer features than intervals that are thickly-bedded and have a lower silica to mudstone ratio. Mechanical layer boundaries also occur at a number of thickness scales and terminate different size structural features, but predominantly occur at distinct changes in stratal stacking pattern. We have also found that thick-bedded dolomite horizons and thin volcanic tuff can be effective mechanical layer boundaries.
AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013