STRUCTURAL EVOLUTION OF THE BUENA VISTA AND ELK HILLS ANTICLINES AND HYDROCARBON TRAPPING POTENTIAL (SOUTH SAN JOAQUIN BASIN, CALIFORNIA) Radu Girbacea Rock Fracture Project, Department of Geological and Environmental Sciences, Stanford University, Stanford CA 94305-2115 and Occidental Oil and Gas Corporation P.O. Box 27757 Houston, TX 77227-7757 e-mail: [email protected]

Introduction Andreas Fault. For exploration activities, the associa- The goal of this study was to provide a structural tion of thrusting and wrenching can provide additional model for the Buena Vista (BV) and Elk Hills (EH) structural trapping potential which might be underesti- anticlines (Fig. 1) and to add new insights into timing mated based on the previous structural models. and possible mecha-nisms of trap formation. The interpretation was based on balanced restoration of a Data used cross-section running from north of EH through BV and The data set used for this study consist of: up to the San Andreas Fault in the south. GeoSec2D • one 3-D seismic line (Line 574); was used to visualize and model critical points, as fold • one 2-D seismic line (Line SJ-132); geometry in connection to observed fault shape and slip • wells with picks and dipmeter data; amount, dif-ferences in forelimb/backlimb dips, and • surface geology (stratigraphy and structures). thinning of stratigraphic horizons across anticlines. The well and seismic line location is shown on the GeoSec2D was also used to unfold the studied cross- base map in Figure 2. The orientation of section 1-1’ section in steps corresponding to each stratigraphic top. (which is discussed here) was constrained by the regional This enabled the recon-struction of the incremental SJ-132 amd therefore is not perpendicular to the mean strains, the cal-culation of the amount of shortening, and fold axis orientation. The planned section 2-2’ may of- the prediction of possible structural features required fer additional calibration because it has more surface by the kinematic constraints and by the observed geologic control and runs perpendicular on the regional structural geometries. structural trends. The wells are listed in Table 1; Figure As a general concluding remark, the BV and EH 3 shows the stratigraphic profile and tops as used here. anticlines can be interpreted in 2D as folds related to a deep decollement with flat-and-ramp geometry. The Interpretation space problems observed during unfolding have been The overall geometry down to the Media top (con- attributed to a young episode of wrenching. However, sidered as being the base of Monterey and top of Tem- thrusting was still required during wrenching in order blor) was constructed using well picks, tops mapped on to account for the present fold geometry. Therefore the Line 574 and then by following the geometry of strong overall structural picture can be interpreted as gener- reflectors on Line SJ-132 (Fig. 4),. The following fea- ated due to thrusting followed by transpression. This is tures are visible on this seismic interpretation: a new interpretation for the structures associated with • a high-angle south-vergent fault (F1) developed up the San Andreas fault in the south San Joaquin basin, to the Tulare Fm. south of EH; which differ from both the pure wrenching (Harding, • a north-vergent fault (F2) developed up to the Reef 1976, Nicholson, 1990) and the pure thrusting (Davis Ridge Fm. north of EH; and Lagoe, 1988) models. Based on this interpretation, • thinning and pinchouts of the Reef Ridge Fm. on the EH and BV differ substantially from other features top of EH; located further south and north along the same struc- • thickening of the Monterey and Reef Ridge forma- tural trend, i.e., Wheeler Ridge (Medwedeff, 1992; tions on the southern limb of EH; Mueller and Suppe, 1997), Lost Hills (Medwedeff, 1989; • a change of fold axis vergence form northward to Wickham, 1995), and Kettleman Hills (Bloch et al., southward in EH; 1993). This difference indicates the high variability in • thinning of the Etchegoin and San Joaquin fms. on strain behavior and kinematic style along the San the northern limb of BV; Stanford Rock Fracture Project Vol. 11, 2000 P-F-1 • a high-angle north-vergent fault (F3), north of BV; Considering the rather small amount of offset mapped • a tight fold and a rapid shallowing of all tops to- on faults F1, F2, and F3, no folding mechanism tested ward the San Andreas fault south of BV; (i.e., fault-bend, fault-slip, and fault-propagation) is ca- • a north-verging reverse fault (F4) was inferred at pable of creating the observed folds. Furthermore the BV to account for the abnormal thickness of the fact that fault F1 is traced off the kink observed in the McDonald Fm. as observed in wells 25P-10D and Etchegoin and San Joaquin fms. on the south flank of 723-9D, and to explain the sudden change in dip of EH proves that the folding cannot be related to fault F1. the top McDonald on the northern flank of BV. Therefore, I suggest that the bulk folding was Several forward models were constructed in GeoSec2D achieved due to a deeper detachment fault with a flat in order to visualize the structural and depositional pro- and ramp geometry, while the faults F1, F2, and F3 are cesses which might have caused the observed features. in fact very young features which have a little impact Table 1 Wells used in this study (CWN-common well name: WO-well operator), Section 1-1’

Section 1-1’ TD No. CWN API WO FEET 1 341-18040293089900 CHEVRON 14011 213040293090100 SUPERIOR 14123 3 31-5040296967000 QUINTANA PROD. 16200 4 56X-10040295293200 BENDER E. A. 13531 5 55-15S040293747400 TENNECO 11315 6 52X-24040298008300 UO-NPR1 12020 7 5-321-26S040294529600 UO-NPR1 11478 8 64-26S040292582500 UO-NPR1 10641 9 316-26S040296510200 UO-NPR1 11000 10 5-377-34S 040295891600 UO-NPR1 11950 11 3-88-3G-RD1 040296865301 UO-NPR1 10000 12 326-9G 040296266700 UO-NPR1 11500 13 USTAN-PO 040296547900 PORTS-OF-CALL OIL 13600 14 1B&N 040296273100 OXY 13770 15 4-7-33G 040290247600 CHEVRON 5687 16 1-1-3D 040290272900 CHEVRON 5650 17 723-9D 040300030600 CHEVRON 11913 18 25P-10D 040291120500 HONOLULU OIL 14622 19 S-2-22D 040290316500 VISTA GRAN 7026 20 543X-27D 040294494700 CHEVRON 7728 21 52-34D 040290764400 CHEVRON 8004 22 401 040296471500 MOBIL 9784 23 1 040293606200 UNKNOWN 8641 24 2 040293541100 OCCIDENTAL 9005

P-F-2 Stanford Rock Fracture Project Vol. 11, 2000 Section 2-2’ TD No. CWN API WO FEET 1 USTAN-PO40296547900 PORTS-OF-CALL OIL 13600 2 1B-20G G201B UNION OIL 9542 3140292385100 UNKNOWN 2404 4 33X-3040291741000 UNKNOWN 10030 5240293919200 UNKNOWN 5011 6 518X-7D40298356600 CHEVRON 8993 784029058100 CHEVRON 3129 81440292595100 INDEPENDENT EXPL. 6030 95840292949400 SUPERIOR 14504 10 1-33 40296023200 TERRA RESOURCES 11011 11 78-31 40291537800 ARCO 11438 Coles Levee Deep Wells (used for stratigraphic control) TD No. CWN API WO FEET 1 71-1040292930900 Marathon 12486 2 22X-1040297696900 Channel Expl. 13522 3 26-2940296065000 Tenneco 17978 4 67-29T40290136500 Arco 17895 on the final fold shape. The inferred deep detachment B. These are fault segments originating from a deep fault is likely to have been originated within the ductile strike-slip fault and thus composing a flower structure. unit of the Kreyenhagen shale, which is also a potential The unfolding results presented in the next chapter in- source rock in this area. The high-angle geometry of dicate that probably this is the case. faults F1, F2, F3 suggests two possibilities: A. These are out-of-sequence faults because an ini- O tial thrust would propagate at angles lower than 45 (ac- Cross-section restoration and cording to the Mohr-Coulomb failure criterion). This angle is affected only if the initial fault becomes locked unfolding while the shortening continues—a case in which the de- The results of the cross-section restoration and un- formation is accommodated by out-of-sequence high- folding are shown in Figure 5a. The unfolding was done angle faults. The out-of-sequence faults are likely to root in six steps, by flattening the main stratigraphic tops, into the Kreyenhagen Fm., because this is a dominantly i.e., Tulare, San Joaquin, Etchegoin, Reef Ridge, shaly unit. Monterey, McDonald, and Temblor. The observed and In this model the locking of the initial fault is cru- inferred features are described below: cial; this might have happened in the study area due to • Step 1, top Temblor. The section has an initial length several possible reasons, such as: of 46.25 km and it shows the basin containing three rela- • changes in rheology toward the basin center due to tively elevated areas (two at BV zone, one at EH), which lithologic heterogeneities; were interpreted as horsts separating deeper areas in • approaching a structural high which acts as a but- between. The space problems (i.e. material missing) seen tress against fault propagation (in this case perhaps the at the northern end of the section indicates that the area Bakersfield arch?); was under extension from this stage to the next one. • decrease in pore pressure with decreasing depth due The normal faults are interpreted as being listric, there- to several ramping episodes. fore a basal extensional decollement is inferred; Stanford Rock Fracture Project Vol. 11, 2000 P-F-3 Channel Expl. Tenneco Arco 12486 13522 17978 17895

• Step 2, top McDonald. The section expands to 47.68 propagating in the basin as a triangle zone. The sole km by 4.35% stretching, this causing further normal thrust of this block can be the same fault causing faulting and deepening of the existing grabens; (through its ramps) the folding at BV and EH, while This extension correlates with an Oligocene-Early the roof fault is a back-thrust. The final interpreta- Miocene extensional event documented in the San tion is shown in Figure 6. Joaquin Basin (Davis and Lagoe, 1988). According to The change of fold axis vergence observed at EH Thor Nielsen (pers. comm.) this extension is likely to (for northward to southward) is probably the result of be a gravity effect associated with regional isostatic uplift different timing of slip along faults; the initial north ver- due to the subduction of the hot, buoyant mid-oceasnic gence is due to slip on the decollement ramp and fault ridge of the paleo-Pacific Plate. F2, while the onset of transpression and the reverse fault • Step 3, top Monterey. The section is shortened by F1 induces fold growth towards south. 3.51% to 46.06 km length. The basal extensional The present structural regime at BV and EH ap- decollement is inverted starting from the south. Normal pears to be mainly thrusting as indicated by drilling- faults at BV are inverted, and together with a ramp induced earthquakes (Occidental of Elk Hills data), present in the decollement, they cause fault-bend fold- combined with a small amount of wrenching as proved ing, while further to the north at EH extension contin- by borehole breakouts (Occidental of Elk Hills data). ues; The combination of thrusting and wrenching is due to • Step 4, top Reef Ridge. The section experiences the the rheological proprieties of the San Andreas Fault. The maximum rate of shortening, by 16.25% to 39.62 km fact that this is a weak fault (i.e. with low friction) en- length. Folding is active at both BV and EH above two ables the deformation associated with the convergence ramps present in the decollement horizon. The pre-ex- between the North American plate and the Pacific plate isting normal faults can be inverted, but they do not show to be partitioned into two strain components: a dextral significant offset. The reverse fault shown in the Tem- strike-slip component of 35mm/y, active over a 5-20 km blor Fm. at EH is required to accommodate space prob- wide zone along the San Andreas Fault; and a pure thrust- lems within the anticline core; ing component of 4mm/y, inducing folding and reverse • Step 5, top Etchegoin. There is 4.09% shortening faulting in an area up the 200 km away form the San for a section length of 38.06 km. Folding continues at Andreas Fault (Mount and Suppe, 1987) (Fig. 7). The both BV and EH above the two ramps in the decollement. EH-BV area can be considered as situated at the bound- The northern end of the section again shows space prob- ary between these two strain zones, with more influ- lems, this time due to material moving into the section. ence coming form thrusting zone. This is interpreted as the onset of wrenching, due to According to this structural model both BV and EH change from a compressional to a transpressional re- anticlines are hangingwall folds related to ramps devel- gime. This interpretation matches the regional tectonic oped in a deep (initially extensional) decollement. Be- models for central California and south the San Joaquin cause these two folds act as structural heterogeneities, basin (Powell and Weldon, 1992), which take into con- the wrenching associated with the young regional sideration the Mendocino triple junction that passes by transpression has been preferentially concentrated at EH this area and the onset of dextral wrenching along San and BV through high-angle faults with a flower struc- Andreas at around 5-6 Ma (see also Fig. 5b for the in- ture-like geometry. Thus, fault F1 is a post-Reef Ridge ferred tectonic events based on this cross-section resto- transpressive structure; fault F2 (post-McDonald) is ration); mainly reverse, but it could have acquired a wrench com- • Step 6, top San Joaquin. The section shortens by ponent too during the transpressional phase; Fault F3 is 3.48% to 36.78 km. Folding above the ramps continues a young transpressive structure, while fault F4 is a post- at BV and EH, with a wrenching component active at McDonald reverse fault. EH (as suggested by surplus material coming into the section); Implications for hydrocarbon • Present geometry shows a 32.9-km-long section exploration (11.79% shortening from the previous stage). Thrusting This structural model predicts the presence of anti- above basal ramps continues at BV and EH, but wrench- clines along the entire stratigraphic column down to the ing is still required (probably at both BV and EH, as the inferred decollement ramps. GeoSec2D cannot predict seismic data may indicate) to accommodate the surplus the depth of this decollement, but if this has propagated material from the previous step. South of BV the struc- from the Cretaceous units up to the Kreyenhagen, then tural complexity increases and the observed features the Temblor Fm. at both BV and EH becomes a viable (especially the tight fold south of BV and the steep north- exploration target. The impact of the young high-angle ward dips) are related to an uplifted basement block (supposedly) wrench faults identified at BV and EH is P-F-4 Stanford Rock Fracture Project Vol. 11, 2000 rather difficult to asses; depending on their lateral and Castillo, D. A., Zoback, M. 1994. Systematic variations in vertical slip amount and lithologies they affect, these stress state in the southern San Joaquin Valley; inferences faults can act either as hydraulic conduits or barriers. based on well-bore data and contemporary seismicity. AAPG Bull.; v. 78, no. 8; p. 1257-1275. Further studies Davis, T. L, Lagoe, M. B. 1988. A structural interpretation of major tectonic events affecting the western and southern The structural interpretation of the Elk Hills and margins of the San Joaquin Basin. Pacific Section SEPM Buena Vista anticlines can be improved by interpret- Field Trip Guidebook, v. 60, p. 65-87. ing other cross-sections parallel to section 1-1’ and Harding, T. P. 1976. Tectonic significance and hydrocarbon comparing the amount of shortening and the struc- trapping consequences of sequential folding synchronous tural style. This comparison, combined with inter- with San Andreas faulting, San Joaquin Valley, Califor- pretation of isopach and isochore maps, can help the nia. AAPG Bull., v. 60, no. 3; p. 356-378. 3-D structural restoration and balancing and the quan- Medwedeff, D. A. 1989. Growth fault-bend folding at South- tification, both in time and space, of the thrusting east Lost Hills, San Joaquin Valley, California. AAPG Bull., v. 73, no. 1, p. 54-67. and wrenching components. Medwedeff, D. A. 1992. Geometry and kinematics of an active, laterally propagating wedge thrust, Wheeler Acknowledgements Ridge, California, pp. 3-28 in Shankar Mitra and I thank Dan Szymanski from Occidental of Elk Hills George W. Fisher, eds., Structural Geology of Fold for permission to release these results to the RFP work- and Thrust Belts. shop participants. I also thank Dan and all the colleagues Mueller, K and Suppe J. 1997. Growth of Wheeler Ridge from Elk Hills—Lesley Cline, Misty Garner, Theresa anticline, California; geomorphic evidence for fault-bend Knox, Judi Petersen, Heather Van Arkel, Dave Boot, folding behaviour during earthquakes. Special Issue: Edgar Camacho, Dave Fowler, Don Greenfield, Jack Fault-related Folding, Jour. Structural Geology, v. 19, no. 3-4; p. 383-396. Grippi, Richard Lewis, Jesse Lomask, Bill Long, Dave Nicholson, G. E. 1990. Structural overview of Elk Hills, pp. Miner, Pedro Romero—for helping and motivating me 133-140 in Structure, Stratigraphy and Hydrocarbon during this study. Occurrences of the San Joaquin Basin, California. Pacific Section SEPM FieldField Trip Guidebook, v. 64. References Powell, R. E., Weldon, R. J. 1992. Evolution of the San Bloch, R. B., Von Huene, R., Hart, P. E., Wentworth, C. M. Andreas Fault. Ann. Rev. Earth Planet. Sci; v. 20; p. 431- 1993. Style and magnitude of tectonic shortening normal 468. to the San Andreas Fault across Pyramid Hills and Wickham, J. 1995. Fault displacement-gradient folds and the Kettleman Hills South Dome, California. Geol. Soc. Amer. structure at Lost Hills, California (U.S.A.). Jour. Bull., v. 105, no. 4, p. 464-478. Structural Geol., v. 17, no. 9; p. 1293-1302.

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Diablo-Temblor Range Sierra Nevada

San Joaquin Basin

San Andreas Fault

Bakersfield Elk Hills N Buena Vista

20 miles

Figure 1. Location of the study area in the south Joaquin Basin.

EH-BV Cross-Section: Change in Length

Transpression Compression Extension ? Length (km)

Figure 5b. Tectonic events in the south San Joaquin Basin as inferred from restoration of the Elk Hills-Buena Vista cross-section.

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Line SJ-132 N 341-18 13 3 Miles

Section 1-1’

Qa 31-5

Elk Hills

56X-10 Line 574

55-15S

52X-24 5-321-26S 934-29R 26-29 TD 22847’ 316-26S 64-26S 67-29T

5-377-34S 9-354-4G-RD1 TUL 3-88-3G-RD1 71-10 326-9G 22X-10

Buena Vista USTAN-PO 1B-2G

33X-30 Qa

4-7-33G 1-1-3D 1B&N 25 10 35 15

5 TUL 723 25P-10D

Qa 518X-7D 8 Cross-Section 2-2’ S-2-22D TUL 58 1 14 TUL 4 52 10 543X-27D 35 50 RR 24 52-34D San Andreas Fault 1-33 ANT 78-31 50 Qa ANT DEV 60 17 401 50 50 1 Qa

TUL 25 25 2

Temblor Range

Figue. 2: Base map with the well and seismic lanes location. See Fig. 3 for stratigraphic names

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Time Intraform ational Period (Ma) Formation Tops Color Picks Holocene Alluvium Qa unconformity Pleistocene Tulare Tul 1.7 unconformity SNJO

Pliocene San Joaquin SJ SCALEZ 5.3 /EC G N Etchegoin Et CLLM_2 /R FD G Reef Ridge RR Upper 7 NA/N_PT AA - Monterey A shale Upper Stevens Sd. 10.8 BCA - intra-Stevens Ant Lower Antelope Sh.

Antelope Sh. McD PG Miocene McDonald Sh. DA - Monterey D shale Monterey LMt DVW

Lower Devilwater, Gould Sh. Monterey MEDIA

Media Sh.

unconformity Carneros Sd.

Tmb Upper Santos Sh. Lower Middle Aqua Sd. 25 Lower Santos Sh. Oligocene Temblor Phacoides Sd. Cymric (Salt Creek, Pleito) 38 TUMEY Tumey Tum KRNHAGN /PT_R C KS Kreyenhagen Eocene Krn Point of Rocks Sd. unconformity Undiferentiated Eocene 55 unconformity Paleocene Undiferentiated Paleocene 67 unconformity Upper unconformity 100 unconformity Lower Franciscan Basement Bsm Cretaceous

Figure 3. Stratigraphy and tops/picks for BV-EH cross-sections

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0 1000 -1000 -2000 -3000 -4000 -5000 -6000 Meters Feet 3000 1000 -1000 -3000 -5000 -7000 -9000 -15000 -19000 -11000 -13000 -17000 Line SJ 132 (SP 2600) (SP 132 SJ Line 341-18 13 1 1-23 122 31-5 a Q Line 570 Line 56x-10 55-15S 52X-24 67-29T 26S 5-321- 64-26S 26S 316- F1 F1 -34S 5-377 9-354- Elk Hills Elk 4G-RD1 3-88- 3G-RD1 Tul PO 326-9G F2 1 Mile 1 N USTAN- a Q F3 4-7-33G 1-1-3D 723-9D 25P-10D Buena Vista Buena Tul S-2-22D 543X-27D Figure 4. Seismic interpretation along Elk Hills and Buena Vista anticlines. See Fig. 3 for stratigraphy tops. F4 52-34D a Q 401 332-6K 2 S NA McD DVW SNJQ Media RFDG ECGN

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Present length 32.90 km (11.79% incremental shortening,Pin Line 44.92% total shortening between Top McDonald and present) BV E H 1 Mile Present F3 F1 F2 F 4

Top San Joaquin 36.78 km (3.48% incremental shortening)

Step 6

Space Problems => Wrenching? Top Etchegoin 38.06 km (4.09% incremental shortening) Step 5

Top Reef Ridge 39.62 km (16.25% incremental shortening) Step 4

Top Monterey 46.06 km (3.51% incremental shortening)

Step 3

Basal Compressional Detachment

Top McDonald 47.68 km (4.35% stretching)

Step 2 Basal Extensional Detachment Space Problems => Extension?

Top Temblor 46.25 km initial length

Basal Extensional Detachment

Step 1

Figure 5a. Folding/faulting history of the studied cross-section

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0 1000 -1000 -2000 -3000 -4000 -5000 -6000 Meters Feet 3000 1000 -1000 -3000 -5000 -7000 -9000 15000 - -19000 -11000 -13000 -17000 341-18 Line SJ 132 (SP 2600) (SP 132 SJ Line 13 1 1-23 122 31-5 a Q Line 570 Line 56x-10 24 55-15S 52X- 67-29T 26S 5-321- 64-26S 26S 316- F 1 F1 -34S 5-377 9-354- Elk Hills Elk 4G-RD1 3-88- 3G-RD1 1 Mile 1 Tul 326-9G F2 N USTAN-PO a Q F3 4-7-33G 1-1-3D 723-9D 25P-10D Buena Vista Buena Tul S-2-22D 543X-27D F4 52-34D a Q Figure 6. Final structural interpretation of the Elk Hills-Buena Vista cross-section 401 332-6K 2 S NA McD DVW Media SNJQ RFDG ECGN

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San Andreas Fault

θ = 5

Pacific North- Plate American Plate

Figure 7. Strain partitioning along the San Andreas Fault, resulting in a dextral strike-slip com- ponent of 35mm/y and a thrusting component of 4mm/y. The wrencing is active over a 5-20 km wide zone along the fault, while the thrusting component causes folding and reverse faulting in an area up the 200 km away form the fault (after Mount and Suppe, 1987) .

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