Steve Laubach

Lecture SL1: Structural diagenesis: An integrated investigative approach to
coupled chemical and mechanical processes in sedimentary systems

Structural diagenesis is a new perspective for understanding how fracturing, fault growth, compaction, and other mechanical processes interact with chemical processes to govern the attributes of structures and physical properties of sedimentary rocks at high crustal levels in the Earth. Although clearly a characteristic part of the postdepositional changes that affect rock, fractures and faults are commonly neglected in conventional diagenetic studies, which thus miss an essential aspect of the transformation of sediment into rock. Likewise, structural studies, at least those concerned with the low-temperature realm of sedimentary basins, tend to focus on geometry, kinematics, and mechanics. Thermal controls and chemical processes are commonly ascribed limited importance, if considered at all. Yet much can be learned about chemical diagenesis by reference to structure, and vice versa, and chemical and mechanical processes may be coupled in many ways. There are undoubtedly some important feedbacks between them. Cross-disciplinary research is beginning to address some of these issues, but systematic student training in principles of both structure and diagenesis is the key to unlocking scientific knowledge about a part of the Earth’s interior that is of great intrinsic and practical interest. These challenges call for a merger of disciplines and a new training paradigm in sedimentary geochemistry and structural geology.

Lecture SL2: Structural complexity in structurally simple fractured reservoirs and reservoir analogs: Applications of structural diagenesis to predicting and assessing reservoir quality in unconventional and fractured reservoirs

Unconventional and fractured reservoirs present many challenges to successful exploration and production owing to subtle structural complexity that is nearly impossible to diagnose using conventional approaches. Many attributes that are challenging to discover in buried rocks are present in exceptionally well preserved outcrops in Mexico, Scotland and Wyoming. These outcrops provide insights into the patterns and origins of reservoir heterogeneity from the regional to the interwellbore scale. These fractured reservoir analogs have abrupt differences fracture sizes, clustering, and porosity preservation variability over vertical distances of a few meters to kilometers and lateral distances of 10s of meters. Heterogeneity results from interplay of stratigraphy, structure, and diagenesis during fracture growth and aging. These rocks provide a rare opportunity to compare what can be sampled using core with attributes that cannot readily be measured using core-based methods. Outcrop analogue studies can improve understanding of fracture distribution in the subsurface. In particular, outcrops may reveal patterns of and controls on fracture intensity and fracture openness that are key determinants of producibility.

Using imaging methods, and diagenetic and mechanical modeling we show how this heterogeneity likely arises. Basinal fracture systems reflect interactions among mechanical and chemical processes integrated over geologic timescales. In the subsurface (>1000 m), where fractures are new rock surfaces created in the presence of hot, reactive fluids, fracture size distribution, aspect ratio and porosity distribution, spatial arrangement, and sensitivity to effective stress changes are among the fracture attributes modified by diagenetic reactions. Results imply that mechanical and chemical feedbacks resulting from cementation in fractures and surrounding rock mass during and after fractures form influence size and shape of fracture pore space. We show that diagenetic history is a prime control on persistent porosity in fractures in basins and thus their capacity to conduct fluid or to create a perceptible seismic response. Even structurally simple reservoirs can have great fracture, and thus fluid flow, heterogeneity.

Laubach, S. E. and Ward, M. W., 2006, Diagenesis in porosity evolution of opening-mode fractures, Middle Triassic to Lower Jurassic La Boca Formation, NE Mexico. Tectonophysics 419, 75-97.

Laubach, S.E., Olson, J.E., and Gale, J.F.W., 2004, Are open fractures necessarily aligned with maximum horizontal stress? Earth & Planetary Science Letters, v. 222, no. 1, p. 191-195.

Laubach, S. E., 2003, Practical approaches to identifying sealed and open fractures, AAPG Bulletin, v. 87, No. 4, (April 2003) p. 561-579.