SCEC Award Number 14118 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Using Borehole Data as a Direct Measure of Stress Directions and Variability to help Constrain the Community Stress Model of Southern California
Investigator(s)
Name Organization
Joann Stock California Institute of Technology
Other Participants Dr. Deborah E. Smith, Ph.D. Staff Scientist
SCEC Priorities 2d SCEC Groups SDOT, FARM, Geology
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 Oriented multiple-arm caliper logs from drill holes in the Los Angeles basin were interpreted for principal horizontal stress directions by using borehole breakouts from near-vertical sections of hole. The well logs were obtained from various providers including oil companies, service companies, and oil field operators. The borehole breakouts are identified as elliptical zones seen in the caliper data, where tool rotation stops and the minimum caliper diameter is equal to the drilled diameter, following criteria updated from those of similar studies. We can evaluate variations in the stress field both horizontally and vertically, and compare these to stress directions inferred from earthquake focal mechanisms. For the Inglewood Oil Field, currently our densest data set, we find evidence for variation of the direction of SH (greatest horizontal principal stress) over horizontal distances less than 1 km at depths from 2-3 km. This variation is over a smaller scale than what is envisioned for the current community stress field models. Yet, the SH direction appears to be consistent within individual blocks where these are closest to the fault. We relate these variations to stress changes caused by the active Newport-Inglewood fault and possibly other faults active within the fault-cored anticline. These observations in general provide a useful tool for comparison with the larger scale stress field models.
Intellectual Merit These results provide measurements of the direction of principal horizontal stress SH in additional areas of Los Angeles basin for which constraints had not yet been directly obtained from boreholes. High density observations in one oil field indicate small scale spatial variations in stress directions over distances < 1 km. We see rapid spatial changes of stress direction across one active fault system. This will be important to further quantify for evaluating earthquake ruptures in regions of complicated geological structures such as flower structures and multi-stranded fault zones.
Broader Impacts This project allowed three female Ph.D.-level researchers to collaborate together and mutually expand their expertise on borehole geophysics, stress field variations, and regional tectonics. One of the researchers (Persaud) is a member of a racially underrepresented group. Our interactions with the database managers at the California Division of Oil, Gas and Geothermal Resources have helped them to understand what forms of digital data and metadata are needed in the data base for scientific use. We expect that this will have a long-term effect of making the data base eventually more useful not just for us but for other researchers.
Exemplary Figure Figure 6. Stress variability in the Inglewood Oil Field (see Figure 4 for location). Map shows the Newport-Inglewood fault system (red lines from California State Fault Map, 2010), with locations of 24 drill holes (green numbers) with data provided by one operator. SH (maximum horizontal stress) directions (purple) were determined from breakouts measured between 2000 and 3000 meters depth in drill holes. Black dots: drill holes with few or no breakouts. Gray dots: drill holes with breakouts. Dots are sized according to total length of breakouts seen in the drill hole. Largest to smallest gray dots: > 17 meters of breakouts, 8-16 m of breakouts, 2-7 m of breakouts, or less than 2 m of breakouts. Length of purple line reflects the consistency of the breakout azimuth in the drill hole, with longer purple lines representing tightly clustered breakout azimuths and shorter purple lines reflecting more scattered breakout azimuths in the drill hole. Wells 4, 17, 6, and 5 show SH azimuths of 35°, 29°, 15°, and 11°, respectively, suggesting that the stress field close to the active fault has a NNE direction of SH but with some degrees of variation over a small scale. The narrow block within the flower structure appears to be characterized by E-W compression in holes 1 and 3. We are still awaiting additional data from hole 2 to determine if its SH direction is NNE like the other holes nearby, or E-W like holes 1 and 3 within the central block of the flower structure. Figure made by J. Stock.
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