MECHANICAL STRATIGRAPHY OF THE WARSAW AND SALEM FORMATIONS, ST. LOUIS, MISSOURI: PRELIMINARY OBSERVATIONS AND INTERPRETATIONS

Fractures play an important role controlling fluid flow within many water and hydrocarbon reservoirs. The purpose of this project is to develop a predictive model for fracture distribution, connectivity and intensity in Middle Mississippian strata of the St. Louis, Missouri, area.

The main observations and preliminary results include: 1) Fracture bed perpendicular length and spacing are primarily controlled by lithology and bed thickness; thick grainstone units have longer and more widely spaced fractures than thin mudstone/wackestone beds. 2) In some grainstone units, small-scale bedforms (e.g. cross bedding) create mechanical sub-units with shorter length and more closely spaced fractures. 3) Lateral facies changes within depositional sequences and complex geometries within facies create mechanical stratigraphy heterogeneities. 4) Depositional genetic boundaries are generally mechanical unit boundaries. 5) Relations between carbonate beds and chert layers or nodules demonstrate different mechanical strengths of the different units. Where chert is intensely fractured, fracturing is localized in adjacent grainstones, whereas in mudstones to wackestones the fracturing in the chert extends out into the carbonate units. 6) The observation that these blasted road cuts show consistent and identifiable fracture patterns implies that induced fracturing is controlled by the same mechanical properties as natural fractures.

The initial results show how fracture distribution within mechanical units is predictable within a framework of sequence stratigraphy, and depositional and diagenetic history.