Mechanical stratigraphy of the Cretaceous limestone in Isfahan region, Iran

Farzipour Saein A., Beygi S., Tajmir Riahi Z., Vatandust M.

Department of Geology, Faculty of Science, University of Isfahan, Isfahan, Iran

Asaein@gmail.com

Macroscopic fractures are typical of deformed brittle rocks. It is well known that under the same boundary conditions (confining pressure, temperature, strain rate), the rheology and, therefore, the degree of brittleness of a rock is sensitive to lithology and grain size. Another important aspect to be addressed when studying fractured reservoirs, especially in the case of carbonate rocks, is the relations between fractures and stratigraphic features such as thickness, facies, sedimentary cycles and diagenesis (mechanical stratigraphy). The mechanical stratigraphy can be defined as the stratigraphy of the mechanical units. These are sequences of strata that deform similarly and are analogous to facies sedimentary stratigraphy, which are sequences of strata representing a similar depositional environment. The term mechanical stratigraphy has been introduced to describe layering that reflects variations in deformation type and intensity caused by petrologic, petrophysical and lithological changes, (The stratigraphic features that control fracture initiation and termination within a sequence of sedimentary rock strata define mechanical stratigraphy of the sequence).

Recent studies on mechanical stratigraphy show that the link between facies, diagenesis and fracture patterns not only exists, but it is fundamental in flow characterization of carbonate reservoirs. Actually, an interdisciplinary approach between sedimentologists and structural geologists is a key to guide fracture modelling and to characterise patterns of flow conduits in carbonate rocks.

This work aims to explore the link between composition thickness- facies, sedimentary cycles and fracturing within a potential carbonate reservoir through the analysis of the outcropping cretaceous carbonates of the Isfahan region in order to find a model for cretaceous rocks in other areas that are not directly measurable.

A special focus is given to the relationship between mechanical boundaries and sequence stratigraphic boundaries in three different location:

1) kolah ghazi area (South eastern of Isfahan) 2) Dizlu (North eastern of Isfahan)

3) ghahjavarestan (Eastern of Isfahan)

Cretaceous rocks have been outcroped in these sections. A complete Stratigraphic study has been done in this area. The structural studies were done in order to define fracture characteristics (fracture typology, geometry and attitude), fracture network properties (organization in fracture sets, hierarchy, length, spacing, density) and their correlation with stratigraphy.

We acquired fracture data by measuring the orientation (strike and dip), the spacing and the length of fractures on the sections which are nearly vertical. The data were acquired by using two methods: sample lines and inventory areas. We analyzed fracture data by using statistical plots and diagrams.

The analysis of these sections demonstrates that a fracture hierarchy exists in relation to the sequence stratigraphic hierarchy. The majority of fractures terminate at genetic unit boundaries or the internal flooding surface that separates the transgressive from regressive hemicycle. Fractures that do not terminate at geneticunit boundaries or their internal flooding surface terminate at lower order sequence stratigraphic boundaries or their internal flooding surfaces. Secondly, the fracture spacing relates well to bed thickness with increasing bed thickness a scatter from the linear trend is observed. This study illustrates how integrating sedimentology and sequence stratigraphic interpretations with data on structural kinematics can lead to refined predictive understanding of fracture attributes.