This work includes new interpretations of shallow offshore geologic structure between Point Arguello and the Santa Maria River within California’s 3-mi coastal limit. These interpretations are based on multi-sensor high-resolution seismic reflection data collected during January and February 1986. Water depths within the survey area range from 16 m (50 ft) nearshore to 70 m (230 ft) 3 mi west of Point Arguello. The sea floor slopes between 0.3 and 0.5° south-westward. The thickness of unconsolidated Quaternary sediment in the survey area ranges from 0 (bedrock outcrop) to almost 50 m (165 ft) off Point Arguello. The survey area crosses the boundary between the northwest-trending Coast Ranges and the east-trending Transverse Ranges. The onshore faults and folds can be traced offshore in the seismic sections. From north to south, these faults include: (1) Pezzoni-Casmalia-Orcutt frontal fault, (2) Lions Head fault, (3) Santa Ynez River fault system, (4) Lompoc-Solvang fault, (5) Cañada-Honda fault, and (6) several unnamed faults offshore Point Arguello. These faults are tentatively classified as potentially active because they do not offset a Pleistocene erosion surface and the Holocene unconsolidated sediments overlying that surface do not show offset in the seismic records. Although the faults are tentatively classified as potentially active, they may be seismically active as suggested by the limited earthquake data in the area. The seismic data show that the north-northwest-striking Hosgri fault zone decreases in both vertical and right-slip displacement toward the south. In the northern and central parts of the survey area, the fault zone consists of two subparallel branches. In the south, near Purisima Point and near the boundary between the Transverse and Coast Ranges, the north-northwest strike of the fault zone changes toward the east and the fault zone shows splays. Our interpretation of the data is that this area of splays may be the terminus of the Hosgri. If that is correct, then the amount of surface rupture due to earthquakes along this segment of the Hosgri is likely to be small, if surface ruptures occur at all.

Abstract Chevron (as operator for its partners, Phillips, Union Pacific Resources, and Impkemix) discovered the Point Arguello oil field in 1981. The discovery well, the Chevron et al. Ρ 0316 #1, was drilled in federal waters 8.5 mi (13.7 km) south of Point Arguello, California. Delineation drilling (by both Chevron and Texaco) has confirmed the discovery of a giant oil field with estimated recoverable reserves in excess of 300 million bbl of oil. The oil field is located within a small depocenter at the southern edge of the offshore Santa Maria basin. This local depocenter may contain over 15,000 ft (4600 m) of Neogene rocks. The Point Arguello accumulation is trapped in a large north-northwest-trending anticlinal complex and is part of an anticlinal trend of similar Monterey oil discoveries and producing fields within the offshore Santa Maria basin. The primary reservoir is the middle and upper Miocene Monterey Formation, composed of fractured cherts, porcelanites, siliceous mudstones, and dolostones. Calculated fracture permeabilities range up to 3 darcys. Crestal wells have productive capabilities, after acid, of approximately 6000 BOPD. Three production platforms have been installed, and connected by oil and gas pipelines to onshore treatment facilities capable of handling 100,000 BOPD and 60 MMCFGD. Thirty-nine development wells have been drilled. Initial production was scheduled for late 1987; however, permitting delays stalled production from this giant oil field until May 1991.

Abstract The fracture system in the Miocene Monterey section at Point Arguello Field, was studied to find the relationship between fracture intensity and local tectonic structure. The analysis included five studies: seismic structural mapping, seismic amplitude, outcrop, core, and surface curvature. The goal was to develop a predictive and quantifying methodology for the exploitation and exploration of fractured reservoirs of the type found in the Point Arguello field. A detailed structure map on the Monterey chert member was constructed from a 3D seismic survey on a Landmark workstation. The time structure map was depth converted with an accuracy of ±100 feet to the lithologic Monterey chert unit within existing well control. Seismic amplitude studies were performed to relate fracture intensity to a reduction in seismic amplitude across the field. An outcrop study was performed at Lion's Head fault to determine the relationship between the distance to a large fault and fracture density. A statistical evaluation of core analyses for 6 wells by Terra Tek (Salt Lake City) was performed to evaluate the variability of fracture intensity as a function of structure. The surface curvature analysis consisted of treating each fault block as a 2 dimensional curved space (closed-compact 2D manifold in R 3 ). At each point the distance to the boundary (faults) was determined as well as measures of the local curvature. The Gaussian curvature, ellipsoidal curvature and second derivative were used to characterize the curvature. The distances to faults and curvature measures were correlated to core analyses and to each other. Results are consistent with the outcrop and core studies, and indicate an ability to use the topology of a seismically derived structure map to predict fractured reservoir parameters.

Abstract The Point Arguello field is located in federal waters at the western end of the Santa Barbara Channel, offshore California. The reservoir consists of highly fractured fine-grained siliceous rocks of the Miocene Monterey Formation. Nine wells were examined, using cores and Formation MicroScanner (FMS) images, to determine the relationship of fractures to lithology and structure. The Point Arguello structure is a doubly plunging, slightly asymmetric anticline created by horizontal compressive stresses oriented N36°E. Monterey Formation rock types in the Point Arguello field consist of interbedded shale/mudstone, marl, porcelanite, chert, and dolostone. The siliceous rocks are in the quartz phase of diagenesis. Core and outcrop studies show that fracture development is highest in the more siliceous rocks (chert and porcelanite) and is lower in more argillaceous rocks. Fracture development is also higher in thin beds. The Monterey Formation is fractured into three distinct sets of joints oriented relative to the principal horizontal compressive stress direction: extensional, longitudinal, and shear. Extensional fractures have dips ranging from 70° to 90° and strike directions ranging + or -15° perpendicular to the fold axis. Longitudinal fractures have dips that are variable but are often normal to bedding and oriented subparallel (+ or - 15°) to the fold axis. Shear fractures cut the other fractures at an angle and occur as conjugate pairs, with the development of one half of the pair dominating over the other. The orientations of fracture sets are similar to those described in outcrop in the western Santa Ynez Mountains. Fractures are grouped according to their order of frequency for each well, with first order fractures having the highest frequency, and second, third, and fourth order fractures having lesser frequencies. All the wells have one dominant set of fractures (first order) with second and third order fractures occurring at comparative frequency ratios of 2:1 to 25:1. No one fracture type dominates in the order of development, and the development appears to be random across the structure. Extensional fractures are common to all the wells, whereas longitudinal and shear fractures are not. Extensional fractures probably provide a mechanism for connecting other fractures that have developed at all scales, from the microscopic to the megascopic. The result is that extensional fractures may be responsible for draining the reservoir. Fracture analyses were conducted on both core and Formation MicroScanner (FMS) images. Two wells had both types of data recorded from the same intervals. The fracture analyses and bed orientations derived from FMS images compare very well with cores and dipmeter data.

ABSTRACT A closely spaced data set of analog and digital (CDP) seismic reflection lines was used to map the structure of the offshore northwestern Santa Barbara basin and the offshore southern Santa Maria basin. East-west structures of the Santa Barbara margin between the Molino gas field and to the west of Point Conception are linear fold and fault trends offset by minor tear faults and, west of Gaviota, by the South Santa Ynez fault zone. The structures in both mapped areas are Plio-Pleistocene in age and most are active today. Offshore of Point Conception in the northwest corner of Santa Barbara basin the change from the east-west tectonic grain of the western Transverse Ranges and the northwest grain of the offshore southern Santa Maria basin occurs across a narrow, structurally complex zone. This “Disrupted Zone” begins near the State Lands/Federal OCS boundary west of Point Conception and extends to the west for several kilometers. Bordered on the east by the thrust faults of the Point Conception fault zone and on the west by a zone of right-lateral shear, the zone is 4 to 7.5 km in width. Orientations of folds, monoclinal flexures and faults within the zone are highly variable (east-west to north to northwest). Widest west of Point Conception, the zone narrows to the north of Point Arguello and becomes less well defined, It also lies on trend with the southern Hosgri fault zone. An older uplift, the Point Arguello-Conception-Amberjack (?) (PACA) high that trends to the southwest of the headland formed by the points, is overprinted by the Plio-Pleistocene structures. Today the tectonic border of the Western Transverse Ranges, including the Santa Barbara basin, is the “Disrupted Zone”. The older “PACA” high is the Paleogene boundary of the Santa Barbara and offshore southern Santa Maria basins.

The Hosgri fault zone (HFZ) is the name given to the southern section of the major coastal fault in central California. The Hosgri separates Transverse Range structure from offshore Santa Maria Basin structure and is a key element for any tectonic model that includes this economically significant region. Previous published maps have not adequately defined the southern termination of the HFZ, the style of faulting on the HFZ, and the relation of the HFZ to surrounding structures. Using more than 1,500 mi of processed seismic reflection data, we have mapped upper Miocene and Pliocene structure in the region of the HFZ offshore from Point Sal in the north, to Point Conception in the south where the HFZ ends against east-west structures in the westernmost Santa Barbara Channel. In the same area, east-west-trending structures in the western Transverse Ranges north of the channel abut against the HFZ. The HFZ is an oblique right-slip fault along most of its length, but significant changes in the style of faulting are associated with variations in fault trend. North of Point Arguello, the HFZ appears to dip at a high angle in the upper 2,000 m of section and is distinguishable from thrust and reverse faults developed to its west. Between Point Arguello and Point Conception it may be a northeast-dipping thrust. Along its mapped length, east-side-up vertical separation is typical and may be more than 400 m on a Pliocene unconformity. Older horizons show more separation; the lower Miocene is up on the east by almost 1 km off Purisima Point. However, individual en echelon segments of the fault show west-side-up vertical separation where expected in an oblique right-slip fault system. No piercing points were found to define strike separation. Pliocene drag folds indicate dextral slip in Pliocene and later time.

The Sword field is located in the western Santa Barbara Channel approximately ten miles (16 km) west of Point Conception and five miles (8 km) south of the Point Arguello field (Fig. 1). Conoco, Inc. and its partners acquired the Sword leases in 1979 in POCS Sale #48. The water depths range from 1000 ft (300 m) at the north end of the field to 1800 ft (549 m) in the southern portion. Two wells have been drilled on the Unit and one other to north of the Unit boundary in Lease P-0320. The OCS P-0322 #1, drilled in 1982, was the discovery well for the field. The OCS P-0320 #2, drilled in 1985, was a successful delineation well. The OCS P-0320 #1 was drilled in 1983, off the Sword structure, on the south limb of the Point Arguello structure. Although hydrocarbons were tested from

Abstract The Arguello submarine canyon/channel system extends over 300 km from the continental shelf off Point Arguello and Point Conception in southern California westward onto the oceanic crust of the Pacific plate. In the northernmost reaches where the canyon system originates, all stages in the evolution of seafloor morphologic fluid flow features—from pockmarks to gullies to converging rills—are observed, similar to what has been described for the Ascension slope, north of Monterey Bay. These features appear to be active today and are linked to fluid leakage from the underlying hydrocarbon basin. The channel dissects a continental slope that exhibits features consistent with large-scale mass wasting. Upslope scarps may be the source of the morphological feature at the base of the slope previously referred to as the “Arguello submarine fan,” with topographic expressions (e.g., large channel meanders, ridges) that are more consistent with mass transport deposits than with deep-sea fan depositional lobes. The modern canyon crosscuts these deposits and parallels an older, meandering channel/canyon to the west. Modern seismicity along the shelf and slope may have, and potentially still can, trigger landslides on the slope. Seismicity associated with seamount volcanism, past subduction, and Borderland transrotational and extensional processes most likely played a role in stimulating mass wasting. The presence of abundant nearby petroleum suggests that gas venting and hydrate dissociation cannot be ruled out as a triggering mechanism for the slope destabilization occurring today. The canyon/channel continues due south on a path possibly determined by the structural grain of north–south-aligned abyssal hills underlying oceanic basement. At latitude 33°18′N, the channel makes a 90° turn (bend) to the west at the E–W-striking Arguello transform fault wall and develops into a meandering channel system that crosses over abyssal hill crustal fabric. The system ultimately straightens as it continues west before veering north, curving around a thickened crustal bulge at a corner offset in the Arguello fracture zone in complex basement structure, and then finally empties into an 800-m-deep basin depocenter.

ABSTRACT Federal OCS tracts in the offshore Point Arena and Eel River basins, presently scheduled for leasing in early 1989, offer excellent potential for the discovery of significant new oil and gas reserves. Environmental challenges to the upcoming lease sale can be expected; however, Hodel’s newly-proposed 5 year lease plan has taken much of the steam out of the opposition by excluding a large number of chiefly low interest and deep water tracts. The offshore Point Arena basin, which extends northwestward for over 100 miles from south of Point Arena to Cape Mendocino, is underlain by thick sections of strongly folded Cretaceous through Neogene sedimentary strata that bear strong similarities to productive sections in the offshore Santa Maria basin and Santa Barbara Channel. Both Monterey (Point Arena Formation) and Rincon (Gallaway Formation) equivalents were penetrated in the three offshore wells drilled by Shell Oil Co. in the mid-1960’s. Numerous oil shows were reported from fractured, cherty Monterey sections, as well as from sands in the underlying lower Miocene Gallaway section. A wireline test of one fractured chert interval in the P-030 well, located at the southeastern end of the basin, yielded a small amount of 29 degree gravity oil, which could indicate that intermediate to relatively high-gravity Monterey oil may be trapped in some structures within the basin. Certainly, the possibility of another giant Monterey field discovery, on the order of the Point Arguello field, cannot be ruled out for the offshore Point Arena basin. The offshore Eel River basin, which lies north and inboard of the Mendocino triple junction, is presently undergoing a tectonic transition from a forearc to a strike-slip basin. Because of a paucity of offshore well information, the Neogene sedimentary section underlying the offshore, which exceeds 10,000 feet locally, is only indirectly known via seismic-reflection data and extrapolation of the geology from onshore. Thus, structures in the offshore Eel River basin should more appropriately be considered as frontier targets in terms of their exploration potential. Importantly, the offshore Eel River basin extends onshore where more than 85 Bcf of gas has been produced from Pliocene Rio Dell sands underlying the Tompkins Hill gas field. The structural trend of the Tompkins Hill gas field can be followed into the immediate offshore where very large on-trend structures are present in water depths of less than 600 feet. Questions as to the oil-generating potential of the offshore Eel River basin are more difficult to answer. However, clues from offshore seismic-reflection data, onshore and offshore hydrocarbon seeps, and more deeply buried potential source rocks suggest that oil production in the offshore cannot be ruled out.