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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Basement ElevationSubsea value

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Low : -29358.9

Figure 9. Top Arbuckle structure contour map of Oklahoma.

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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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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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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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