Gautam Mitra, PhD, University of Rochester
Basic scientific research on fluid flow through low permeability rock formations, specifically on how fractures affect fluid flow, is increasingly important because of hydrocarbon exploration in shales and mudstones and because such “tight” formations are being utilized as seals for geologic carbon sequestration. Despite considerable research on the geometry and physical characteristics of fracture networks, there is still a lack of understanding about how fluids move through tight rock and it is difficult to predict the conduits that fluids will follow while migrating in the subsurface. Here, we present the beginnings of our work on how fractures affected the flow of fluids through the middle Devonian strata of New York and Pennsylvania and highlight some of the important questions that we will be tackling in the future.
There are many unanswered questions and abundant room for improving our fundamental understanding of subsurface fluid flow processes. The ACS-PRF New Directions grant has allowed us to study some of these fundamental questions using new techniques and to integrate structural geology with geochemistry. One Masters student (completed) and one Ph.D. student (ongoing) have been partially supported on this grant; four abstracts have been published.
Research Focus & Methods
The three major topics we are working on are developing an understanding of how regional structures affect the fracture network present in the Appalachian plateau; evaluating how burial and tectonic history created and altered the fracture network; and determining the relative importance of fractures as conduits for fluids through the subsurface.
Our research has centered on the middle-Devonian Hamilton group, composed mainly of carbonates and siliciclastic black and grey shales that outcrop in the Appalachian plateau of New York and Pennsylvania. These rocks are gently folded and faulted, and display a network of extension fractures (joints) and shear fractures everywhere across this region. Field mapping and sample collection were conducted at 7 locations (outcrop and quarries) in New York and 8 sites in Pennsylvania as well as in 1 core in New York.
Our field studies have focused on identifying, mapping, and categorizing sets of extensional and shear fractures, determining relative ages of fracture sets, and collecting samples of mineralized fractures for laboratory analysis. The methods employed in the field consisted of pace and compass mapping of structures, meter-square grid-mapping (with 10cm subdivisions) of fracture networks, measurement of 3-D fracture orientations, and sampling of vein-filled fractures with a coring drill.
The laboratory methods employed on mineral filled veins consisted of optical petrology, scanning electron microscopy (SEM), fluid inclusion analysis and geothermometry, solution based chemistry, and trace elemental mapping with cryogenic laser ablation ICP-MS.
There are 8 to 10 sets of fractures present within the Hamilton group that can be observed in outcrop and differentiated on the basis of their orientation and physical characteristics. Although the orientations of fractures and number of observed sets vary somewhat, the dominant fracture sets observed throughout all the sites are steeply dipping extension fractures (joints) that strike N-S, ENE, E-W, SSE, and bedding-parallel fractures. Additionally, there are moderately dipping conjugate fracture sets that strike NE and SE. There are also gently dipping shear fractures striking N-S and dipping East and West, and striking E-W and dipping North and South. Only some of the fracture sets contain prominent mineralization: steeply dipping N-S striking vein-filled fractures are the most common. Although timing relationships are complex, we have worked out a general sequence based on cross cutting relationships; there is an early E-W shortening, followed by N-S shortening with some counterclockwise rotation, and a late stage N-S extension.
Optical petrology of veins shows calcite crystals with some pyrobitumen. The presence of larger crystals within the fracture and smaller crystals at the vein wall indicates that crystals with a preferred growth orientation out-compete other crystals, and suggest a history of growth from the walls of the vein towards the center, i.e. syntaxial crystal growth.
Fluid inclusion analysis and geothermometry indicate that methane and degraded bitumen are the most common types of inclusions. Unfortunately it is difficult to determine trapping conditions of fluid inclusions without using aqueous or carbon dioxide inclusions. We are experimenting with alternative methods.
Scanning electron microscopy was useful for imaging the microcracks that cut both along grain boundaries and across calcite grains. The presence of minor amounts of pyrobitumen within these microcracks suggests that calcite mineralization began before all the bitumen was lithified and highlights the possibility that fractures acted as conduits for multiple types of fluids over time. Additionally, some fractures show micro-slickenlines indicating sliding.
We are using solution based ICP-MS to measure trace element concentrations in the shale matrix as well as bulk vein chemistry. This technique provides us with the baseline chemistry of the matrix for comparison with trace elements present within calcite filled fractures.
Cryogenic laser ablation ICP-MS has been an effective method for mapping the trace elemental changes within veins and the shale matrix (see figure in nugget). We have produced several maps highlighting changes in trace elemental concentrations as crystals grew within fractures; the data suggest that veins may have complex growth histories that involve multiple mineralizing events.
We will continue our examination of the fracture network in the Appalachian basin and use geochemical techniques to fingerprint and analyze the veins that fill these fractures. Now that we have characterized the fracture network at the surface (and in 1 rock core), we will compare our findings with fractures observed in 5 more cores (3 in New York and 2 in Pennsylvania). We will also examine the effects of burial history and proximity to regional structures on fracture intensity and characteristics in both outcrop and core, paying close attention to the changes in vein morphology and chemistry. By applying our toolbox of geochemical methods we will continue to study how the fracture fabric in the Appalachian plateau changes and determine if these changes altered the way in which fractures conducted fluids.