Abstract

A study of up to 2.5 km (8000 ft) of gas-bearing shale has shed new light on the relationship between geochemistry and overpressure. Our analysis of a few large geochemical data sets has also identified alternative ways to look at down-hole pressures and has revealed some limitations of currently available tools, such as Rock-Eval pyrolysis below a certain depth in Alberta and Quebec.

The very thick Ordovician Shale of the St. Lawrence Lowlands in Quebec is gas-bearing and prospective. Whereas the prime target is the carbonate-rich Utica Shale, the overlying and thick Lorraine Shale was also studied. Recent Quebec geochemical data from the St. Lawrence Lowlands was analyzed together with hydraulic fracture and reservoir pressure data from the same wells. The first phase of the study focused exclusively on vertical wells and delivered new insights with respect to pressure domains and fracture gradients.

Initial shut-in pressures (ISIPs) measured during hydraulic fracture jobs have been studied per well and per area in conjunction with geochemical data (percentage of methane, ethane, propane, wetness, and carbon isotopes). There is a remarkable correlation between wetness, isotope reversal, and ISIP in each well. Our analysis indicates that the traditional pressure gradient calculation could be challenged because some of the harder and easier rocks to fracture (to initiate breakdown, which depends on SH_min, SH_max, down-hole pressure, well bore orientation, and so on) have seemingly the same fracture gradients.

An isotope reversal for both ethane and propane takes place at a depth where Rock-Eval data begin to become erratic because of low S2 peaks. That depth coincides with the ISIP trends per well intersecting the normal fracture gradient and is related to the limit between a naturally pressured domain and an overpressured domain located below it. Our study indicates that this overpressure/isotope reversal depth could be used to accurately calculate the reservoir pressure at any depth within the overpressure domain.