KATSUBE,T.J., Geological Survey of Canada, G.L. BOWERS,Applied Mechanics Technologies, D.R. ISSLER, Geological Survey of Canada, and J. BLOCH, Scealu Modus

Abstract: Shale texture effect on pore-structure/permeability evolution characteristics: implications for overpressure mechanisms

Recent studies have shown that shale connecting porosity and permeability usually decrease with compaction whereas storage porosity may or may not.This implies that over-pressure developed by compaction disequilibrium may be enhanced in shales that undergo storage porosity reduction. Another implication is that overpressure need not be characterized by enhanced porosity as is commonly assumed.An important question we wish to address is, "what is the role of shale texture in the development of overpressure". Petrophysical and textural characteristics of shales from several sedimentary basins (North America) have been examined in order to learn more about the evolution of shale pore-structure and permeability during compaction, and the resultant implications for fluid expulsion, entrapment and pressure history during sedimentary basin development.

The response of storage and connecting porosity to changes in pressure can be inferred from laboratory measurements of permeability, formation resistivity factor and effective porosity as a function of pressure. Figure 1 shows examples where storage porosity is relatively constant (samples V-7 and V-8) and where it decreases (sample V-4) with increased effective pressure (Pe). These samples are from depths of 5 to 6 km in the Venture Gas Field (eastern Canada). The situation for sample V-4 is one where the compressibility of the storage pores could contribute to the development of overpressure under appropriate stress conditions. For the case of an incompressible storage pore framework, increased fluid expansion is a mechanism for overpressure development. Measurements for North American shales indicate that permeability can attain nano-Darcy (10-21 m 2 ) values at burial depths as shallow as 1 km (Pe ~ 10 - 15 MPa). This implies that conditions for overpressure development can occur over a broad range of depths, depending the pore-structure, texture and overpressure mechanism.

Pore-structure evolution can be divided into three stages: mechanical compaction of diagenetically unaltered shales (stage I), maximum compaction of diagenetically unaltered shales (stage II), and diagenetically altered shales with or without secondary dissolution pores (stage III). Also, shales can be divided into three groups based on their permeability-pressure characteristics (Figure 2) AA (10-20 to 10-18 m 2 ), BB (10-22 to 10-19 m 2 ) and CC (3 x 10-22 to 10-18 m 2 ). Whereas enhanced porosity is commonly associated with overpressure, particularly for Tertiary basins, it need not be diagnostic of overpressure (e.g. stage III), depending on the stage of compaction, pore type and type of overpressure mechanism.

For compaction stage I, sediments with a high matrix content that are deposited under high sedimentation rates could develop undercompacted textures associated with over-pressure. This is because rapid decreases in permeability with burial depth can retard the expulsion of pore fluids during the early stages of burial. For sediments with a high silt-sand content, the amount of matrix material is an important factor influencing the pore pressure history (Figure 3a). Recent work shows that a matrix content of 25 weight percent can reduce permeability to nano-Darcy values. Overpressure resulting from fluid expansion (latter stage I, stage II or III) can be maintained at sufficiently low permeability values. For stage III, if dissolution weakens the storage pore framework, overpressure could develop as a result of storage pore collapse under burial stress (Figure 3c). Pore collapse has been observed experimentally.for diagenetically altered shales, comprised mainly of secondary dissolution pores, with effective porosity values of 12 %.

There is direct and indirect evidence that sheet-like connecting pores develop as a result of overpressure. This increase in connected porosity appears to be small in magnitude, and therefore, probably has little effect on effective porosity but could significantly influence electrical resistivity and sonic velocity. In light of the fact overpressure may be associated with either increased or decreased effective porosity, the effect of connecting porosity variation on electrical resistivity and sonic velocity may be a more reliable indicator for overpressure, compared with the density and neutron log responses to effective porosity variations.