AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Primary Depositional Controls on Shale Gas/Shale Oil Potential in the U.S.: A Global Sequence Stratigraphic Perspective
(1) Neftex Petroleum Consultants Ltd, Abingdon, United Kingdom.
Published estimates of unconventional hydrocarbon resources are typically many times greater than proven conventional reserves, yet exploration campaigns have met with variable success with well production rates varying across the area of a play. Whilst in part related to the application of different drilling technologies, a key control is regional geology and an understanding of the distribution of organic matter, silica and carbonate within a sequence stratigraphic context, along with the regional stress regime, faulting patterns, overpressure and thermal maturity. An increasing focus for US unconventional hydrocarbon companies is to ascertain better well locations in the pre-drill phase, with the aim of reducing the number of low-value wells drilled, and increasing the commerciality of any campaign. This can only be achieved through improved understanding of the subsurface regional geology and stratigraphic context.
The application of our biostratigraphically calibrated, global, 3rd-order sequence stratigraphic Earth Model to these shale gas/shale oil horizons helps constrain and predict regional lithological variations. It also reveals significant depositional differences between individual shale gas/shale oil horizons. For example, the Fammenian Bakken Formation represents multiple stacked 3rd order sequences deposited in a structurally differentiated, dysoxic basin with varying sediment supply. Organic-rich Bakken shale horizons are preserved at the start of each transgression. In contrast, the Givetian Marcellus Shale is composed of one 3rd order sequence, deposited in a basin with a higher sediment supply. Within the Marcellus Shale organic-rich shale horizons are preserved at the start of progradation; both within the early lowstand systems tract and within the overlying highstand systems tract. This sequence stratigraphic approach can ultimately provide a basis for genetically subdividing a shale gas/shale oil horizon, and modelling lithological variation across a basin to help define zones with an improved production profile and the best zones for fracing. A sequence stratigraphic approach will be valuable for exploiting shale gas and shale oil resources in under-explored international areas because it allows us to predict the geographic and stratigraphic distribution of TOC, kerogen type and lithological variability; thereby determining the “sweet spots”.