analysis of induced seismicity and basement rock fluid flowanalysis of induced seismicity and...
TRANSCRIPT
Analysis of Induced Seismicity and Basement Rock Fluid Flow
By: Matthew Tello
Abstract:
Seismicity within the midcontinent United States has increased over the last decade and
is a growing problem for public safety and infrastructure. Several studies provide evidence
linking wastewater injection with the occurrence of seismic events, most recently in Oklahoma.
Earthquakes in Oklahoma range in moment magnitude (Mw) 1.4 to 5.6, and depths of 0.1 to 19.4
km, with wastewater injection of produced fluids occurring at depths up to 4100 m (subsea).
Earthquakes are not widely distributed, but rather relatively concentrated in the central region of
Oklahoma.
To date, very little research has been done on the sediment-basement nonconformity and
the possibility of pressure communication across the nonconformity. This research project is a
crucial first step in analysis of the potential pressure communication across the nonconformity.
The outcome of this project is a detailed map of Oklahoma earthquakes, including depth,
occurrence, magnitude, distance from major populated areas, and geologic formations (especially
depth to basement rock formation), in addition to the location of Class II water disposal wells
and their injection rates and pressures. Sediment-basement nonconformity descriptions from core
and 3D digital core photo-models were documented from the Core Research Center in Denver,
CO. Observations of outcrop nonconformity were completed at Blue Mesa Reservoir in
Gunnison, CO.
These data are used to characterize the basement nonconformity and the possible link
between earthquake occurrence and wastewater injection wells using an ArcGIS database. The
mapping efforts of this project included data from Ohio, Nebraska, South Dakota, and
Oklahoma, but focused on Oklahoma due to its continued earthquake activity.
Map based analysis indicates that the majority of the seismicity occurs in central and
north central Oklahoma, and may be associated with Class II saltwater disposal (SWD) wells and
deep SWD wells injecting into the Arbuckle Formation. The Arbuckle Formation overlies the
basement at the nonconformity. Average injection pressures in the area ranges between 0 and 65
MPa. The majority of the earthquakes, including all earthquakes larger than Mw 3.5, are located
within the Cherokee Platform, the Nemaha Uplift, and the Anadarko Shelf.
Earthquakes > Mw 3.5 have epicenters > 2.15 km deep. Depth to basement in
this region is 0.6 to 2.4 km, verifying that the majority of high magnitude
earthquakes are occurring within the basement rock. Additionally, these
large deep earthquakes trend similarly to the regional tectonic providence
boundaries and their crosscutting extensional faults.
Map data and results from this project will be used as a part of a
collaborative research effort that evaluates the effect of deep wastewater
injection on rock strength, mechanical properties, and mechanical rock
failure.
Introduction
Produced waters associated with oil and gas extraction from
tight formations requires injection into subsurface reservoirs because
they contain dissolved potassium, sodium, chlorine, and minor
amounts of other elements. These brines are separated from the produced oil and gas at the
surface and are injected into disposal/storage reservoirs through a Type II wastewater disposal
Figure 1. Model of oil and gas production with associated UIC well. (Figure retrieved from EPA, 2012)
well (Figure 1) (EPA, 2012). Induced seismicity occurs when injection pressure exceeds the
lithostatic pressure of the formation, resulting in formation of new fractures or by reducing the
frictional stress on existing critically stressed faults (Rubenstein and Mahani, 2015).
Mid-continent earthquake events, such as the Mw 5.7 earthquake that occurred in Prague,
Oklahoma, have increased interest into the causation of earthquake events within states distant to
tectonic plate boundaries and mid-continent seismogenic zones. There has been a gradual
increase in Oklahoma earthquake activity since 1930 up until 2009. Around 2009, the number of
mid-continent earthquakes increases from approximately 50 to 2500 earthquakes per year in
2015 (Figure 2). The majority of these earthquakes are low in magnitude, Mw 2 - 3.5, which are
mostly not felt events and do not cause any structural damage. However, there has also been a
large increase in large seismic events, > Mw 3.5, as well as earthquakes with epicenters at depths
> 5 km, that can be hazardous to the public and infrastructure (Figure 3). For example, the 2011
Pargue, OK Mw 5.7 earthquake caused millions of dollars in damage and injured several people
(Ellsworth, 2015).
Due to uncertainty associated with correlation between depth of injection and earthquake
occurrence, this project explores the possibility of reactivation of unmapped fault zones, pore
fluid pressure communication between sedimentary injection reservoirs and earthquake activity
occurring in crystalline basement. The objective of this project is to create a GIS database that
combines earthquake data in Oklahoma, Ohio, Nebraska, and South Dakota based on depth,
occurrence, magnitude, distance from major populated areas, and geologic formations (especially
depth to basement rock) with the location of Type II wastewater disposal wells, their injection
formation, rates and pressures.
Figure 2. Histogram of earthquake frequency per year from 1970-2015, including cumulative frequency (black line).
Special attention is being paid to Oklahoma due to recent research linking injection of
waste water to earthquake occurrence (Keranen, 2013), the dramatic increase in seismic activity,
and because Oklahoma is injecting into reservoirs that are at the nonconformity.
Figure 3. Graph of earthquakes including depth and magnitude per year 2010-2015.
Additionally, basement and overlying sediment interface was examined using core
samples and outcrops from various locations. The maps built and data collected from this project
will be used as a part of a larger scale research effort that evaluates the effect of deep wastewater
injection on rock strength, mechanical properties and mechanical rock failure. The database will
support further research at Western State Colorado University and by research scientists at Utah
State University, Sandia National Laboratories, and New Mexico Institute of Mining and
Technology.
Geologic Setting
Oklahoma includes several tectonic provinces that were formed by subsidence, arching
and mountain building events (Figure 4). The main areas of petroleum production are the
Anadarko Basin, the Arkoma Basin, the Ardmore Basin, and their associated shelves and
platforms. Although Oklahoma petroleum reservoirs occur in several formations, the majority of
production comes from Pennsylvanian Formations (Boyd, 2002) (Figure 5).
The basement rock of Oklahoma is composed of igneous and metamorphic rocks that
were formed during the Precambrian and Cambrian. From the Late Cambrian through the
Mississippian, Oklahoma was inundated by the sea several times forming limestone, dolomite,
sandstone and shale. Then, during the Pennsylvanian, several episodes of orogeny and
subsidence occurred creating traps and seals for petroleum reservoirs, including the reverse fault
zones associated with the Nemaha Uplift (Johnson, 2008). These reverse fault zones form the
structures that trap and seal the petroleum of the Oklahoma City oil field and is the current and
largest oil producing field in Oklahoma (Gay, 2003). Post-Pennsylvanian time, Oklahoma has
been essentially inactive.
Methods
The focus of this project was to create a database of earthquake distributions and depth to
crystalline basement rocks in ArcGIS.
The ArcGIS database consists of metadata available online in excel spreadsheets from the
United States Geologic Survey, the Oklahoma Oil and Gas Commission (OK OOGC)
(http://www.occeweb.com/og/ogdatafiles2.htm) and formatted shape files from ArcGIS online
(http://www.arcgis.com/features/). Importing data was done using several tools and techniques
in ArcGIS. The borehole location data for all boreholes in Oklahoma, earthquakes, and injection
wells were added using the add xy data tool. Once imported as Lat/Long points, the metadata in
the attribute table of the new files were queried and exported as shapefiles (.shp) to show the
target data including, Class II wastewater disposal wells, wells that drill into the basement rock
and the Arbuckle formation, earthquakes Mw 3.5 or higher, all earthquakes, and wells with
packer depths greater than 1.8 km. Most of the data in these shapefiles came from multiple
sources, which required merging of shapefiles that had overlapping attributes, especially
API/UWI numbers and Lat/Long coordinates, which combined data fields for each point. The
basement structure contour maps were created using the natural neighbor’s tool, which creates a
raster file of basement rock -- wellbore intersection depths relative to sea level using the ground
level elevation as the datum and basement formation top pick within the public database from
OK OGCC as the depth point. The same process was used for the Arbuckle Formation as a
quality control measure. The Arbuckle Formation is the sedimentary formation that lies
stratigraphically on top of the basement rock and is one of several injection formations. The
Arbuckle Formation had several more borehole penetrations, thus it was compared with the
Figure 4. Tectonic provinces of Oklahoma including major fault zones (Campbell and Northcutt, 1998).
Figure 5. Stratigraphic column of the Anadarko Basin, Oklahoma, USA. (http://www.forestargroup.com/assets/minerals/FORE_Anadarko.pdf)
basement map as a structural control. Finally, a map of the tectonic lineaments (Figure 4) was
added as a .jpeg and was georeferenced to correlate the outline of the Oklahoma counties layer.
Core description and photogrammetry was carried out at the USGS Core Research Center in
Denver, CO and fieldwork was conducted at an outcrop in Curecanti National Recreation Area in
Gunnison, CO. The six cores chosen for the study were from Wyoming, Nebraska, South
Dakota, Colorado, and Arizona (Figure 6). All of the cores analyzed in this study drilled into
basement rock and included a nonconformable contact with the overlying sediments. Core
analysis included description of lithology and structural features with an emphasis on the
interface between the basement and overlying sediment. The core samples were documented as
3-D models using AgiSoft. AgiSoft is a photogrammetry program that can be used to render
Figure 6. Core locations.
photographs taken on a rotating table and shooting at every 10 degrees. Two or three rotations
were done at different heights and angles but distance from the sample was kept constant.
AgiSoft uses these photos and renders them as a 3-D model (Figure 7). Outcrop analysis of the
nonconformity was undertaken at Curecanti National Recreation Area at Lake Fork, Blue Mesa
Reservoir overlook and will be used to compare with features seen in core samples. At this
locality, the nonconformity is defined by the contact between the Jurassic Wanakah Formation
and the 1.73 Ma Biotite schist and gneiss (MacLachlan, 1981). Lithology descriptions and
structural analysis including strike and dip of contacts and fractures were done at this locality.
Figure 7. Digital 3D core models of sections at the nonconformity.
Results
The ArcGIS map of Oklahoma includes earthquakes based on depth, occurrence,
magnitude, distance from major populated areas, and geologic formations (especially depth to
basement rock formation) in addition to location of Type II wastewater disposal wells, their
injection rates and depths. To improve the accuracy of the top basement structure contour map, a
separate map of the Arbuckle was created. Where the basement structure contour map was not
reliable, due to lack of borehole penetration data, the points with the lowest recorded depth for
the Arbuckle Formation were used. This provided a template for the possible depth of the
basement because the Arbuckle is the formation that lies on top of the basement, thus the
basement must be as deep or deeper than the deepest recorded Arbuckle depth (Figures 8 and 9).
Approximately 92% of earthquakes > Mw 3.5 have epicenters > 10,000 ft deep, with a
greatest depth of 78,739 ft (Figure 10). In addition, all of this high magnitude seismic activity is
occurring in central and north-central Oklahoma where the basement rock is between 3,000 and
8,000 ft subsea, with the exception of 20 earthquakes that occur where the basement rock is
between 8,000 and 10,000 ft. deep (Figure 11). However, the depths of those earthquakes are all
beyond 16,000 ft. deep. 99% of high magnitude earthquakes are occurring on the Cherokee
Platform, the Nemaha Uplift and the Anadarko Shelf (Figure 11). Note here that the Nemaha
Uplift is an uplifted block that sits between the Anadarko Shelf and the Cherokee Platform. The
structure of the basement rock rapidly deepens to the southwest of the Cherokee Platform and to
the south of the Anadarko Shelf, from 2000 ft. to a maximum depth of approximately 29,000 ft.,
over approximately 185 miles. Basement rocks then outcrop abruptly on the north side of the
Wichita Uplift where the subsea elevation jumps from -29,000 ft. to 1000 ft. over approximately
28 miles.
Figure 8. Top Basement Structure Contour Map, elevations relative to sea level and datum is based on the reported drill floor elevation. Data compiled from reported borehole intersection depths available from the Oklahoma Oil and Gas Commission.
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LegendBasement Contours
Basement ElevationSubsea value
High : 1558.7
Low : -29358.9
Figure 9. Top Arbuckle structure contour map of Oklahoma.
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Arbuckle Contours
ArbuckleDepth from subsea
High : 28857.6
Low : -1453.93
0 25 50 75 10012.5Miles
Figure 10. Depth versus magnitude, Oklahoma earthquakes 1979-2015.
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Figure 11. Basement structure map including all earthquakes and faults.
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µ LegendFaults
Basement Contours
Basement ElevationSubsea (ft)
High : 1558.7
Low : -29358.9
All Earthquakes_1970-2015Magnitude
2.0 - 3.0
3.1 - 4.0
4.1 - 5.6
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In addition to the maps, well injection pressures were investigated by creating graphs of
packer depths versus injection pressures, and included hydrostatic, lithostatic, and EPA fracture
gradients (Figures 16 and 17). One was made for data from 1997-2010, and another was made
for 2012-2015. Although the graph for 2012-2014 shows no cases of injection pressures
exceeding the lithostatic or fracture gradient, it is slightly skewed because not all injection wells
had packer depths or pressures (Figure 16). On the other hand, the graph of 1997-2010 pressures
illustrates several cases where the injection pressure exceeded the lithostatic or fracture gradient
(Figure 17).
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Figure 16. Packer Depth versus injection pressures from 2012-2014. Pressures were recorded monthly for each well, but data points were not separated by well. Wells are colored by year.
Data were also compiled for other states including Ohio, Nebraska, and South Dakota. A
map for Ohio was created because there is both seismic activity and wastewater injection
happening. The map of Ohio includes earthquakes based on depth, occurrence, magnitude,
basement structural contours, location of faults, and location of Class II wastewater injection
wells (Figure 18). More data is needed to evaluate the effects of wastewater injection on seismic
activity in Ohio.
Preliminary analysis of the sediment-basement nonconformity was characterized in cores
at the USGS Core research Lab and an outcrop at Curecanti Nation Recreation Area, which is an
exposed nonconformity between the Jurassic Wanaka Formation and the Precambrian basement.
This highly weathered interface was composed of crushed fine sediments, presumably basement
rock. It also showed several fractures in the basement rock that strike in several directions but all
had consistently steep dips. Core samples are characterized as having mostly weathered
nonconformities and calcite filled micro fractures crosscutting the cores. The samples were
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Figure 17. Packer Depth versus injection pressures from 1997-2010. Pressures were recorded monthly for each well, but data points were not separated by well.
digitally documented as 3D models using photogrammetry for further research or as a reference
when reviewing field and lab notes (Figure 7).
The findings and outcomes of this project are a small part of a larger scale project
involving fluid flow across the sediment - basement interface. The basement and overlying
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LegendOhio Earthquakesmagnitude
2.1 - 2.4
2.5 - 2.8
2.9 - 3.7
Ohio Class II Wells
Precambrian faults
Precambrian 100ft contoursµ
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Figure 18. Map of basement rock structure Ohio, including SWD wells, known faults, and earthquakes
sediment interface plays an important role in fault reactivation. Further work investigating
sediment-basement nonconformities and Arbuckle SWD wells, as well as the sudden increase in
high magnitude earthquakes on the Anadarko Shelf from 2014 to 2015 would compliment this
study.
References Boyd, D. T., 2002, Oklahoma Oil: Past, Present, and Future: Oklahoma Geology Notes, v. 62,
no. 3, p. 97–106. Campbell, J. A., & Northcutt, R. A. (1998). Geologic Provinces of Oklahoma. Proceedings of
the International Conferences on Basement Tectonics Basement Tectonics 12, 225-226. doi:10.1007/978-94-011-5098-9_16
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