GeoScienceWorld

Active Margin Basins

By Kevin T. Biddle

Abstract

“The most distinctive characteristic of the Los Angeles basin“The most distinctive characteristic of the Los Angeles basin is its structural relief and complexity in relation to its age and size” (Yerkes et aI., 1965, p. AI6); however, its very complexity caused no small amount of discussion in designing and naming this volume of the AAPG World Petroleum Basin Memoirs. (See the Foreword for a discussion of the scope of these memoirs.) The series coordinators decided early that the Los Angeles basin should be included in the World Petroleum Basins project because of its interesting geology and importance as a hydrocarbon producer. Initially, the Los Angeles basin was considered for a convergent-margin volume, presumably in recognition of the late-stage shortening that has taken place in the Los Angeles region of southern California. There is little doubt, however, that the Los Angeles basin has formed and deformed within the evolving San Andreas transform system (Atwater, 1970, 1989; Campbell and Yerkes, 1976; Blake et al., 1978; Engebretson et al., 1985; Wright, this volume). There is also little doubt among those who have worked in the area that the initial subsidence of the Neogene Los Angeles basin was caused by extension (Yeats, 1968; Crowell, 1974, 1976, 1987; Wright, this volume). The series coordinators decided, therefore, that to portray the Los Angeles basin as a model for basins formed in convergent-margin settings would be misleading.

The title of this volume, Active Margin Basins, is a compromise, but, like many compromises, this title falls short of completely describing its subject

  1. Page 1
    Abstract

    “The most distinctive characteristic of the Los Angeles basin“The most distinctive characteristic of the Los Angeles basin is its structural relief and complexity in relation to its age and size” (Yerkes et aI., 1965, p. AI6); however, its very complexity caused no small amount of discussion in designing and naming this volume of the AAPG World Petroleum Basin Memoirs. (See the Foreword for a discussion of the scope of these memoirs.) The series coordinators decided early that the Los Angeles basin should be included in the World Petroleum Basins project because of its interesting geology and importance as a hydrocarbon producer. Initially, the Los Angeles basin was considered for a convergent-margin volume, presumably in recognition of the late-stage shortening that has taken place in the Los Angeles region of southern California. There is little doubt, however, that the Los Angeles basin has formed and deformed within the evolving San Andreas transform system (Atwater, 1970, 1989; Campbell and Yerkes, 1976; Blake et al., 1978; Engebretson et al., 1985; Wright, this volume). There is also little doubt among those who have worked in the area that the initial subsidence of the Neogene Los Angeles basin was caused by extension (Yeats, 1968; Crowell, 1974, 1976, 1987; Wright, this volume). The series coordinators decided, therefore, that to portray the Los Angeles basin as a model for basins formed in convergent-margin settings would be misleading.

    The title of this volume, Active Margin Basins, is a compromise, but, like many compromises, this title falls short of completely describing its subject

  2. Page 5
    Abstract

    The Los Angeles basin is a polyphase Neogene basin within the San Andreas transform system between the Pacific and North American plates. The basin was initiated in the mid-Miocene by widespread extension associated with significant strike slip and rotation of the Transverse Ranges of southern California. Late Miocene to early Pliocene extension, which accompanied the opening of the Gulf of California, led to the principal phase of basin opening. The early Pliocene to Recent deformational history of the basin is characterized by shortening associated with the active North Los Angeles fold and thrust system.

    The Los Angeles basin is the richest basin in the world in terms of hydrocarbons per volume of sedimentary fill. Each phase of basin evolution has contributed to the basin's productivity. Some aspects of the basin's history that have affected the occurrence of oil and gas are related to basin-forming mechanisms and can be used to guide thinking in similar settings. Other first-order controls on hydrocarbon occurrence stem from processes that operated on a much larger scale and are not related to basin type. Deposition of thick, high-quality source rock within the Los Angeles basin is the result of such regional controls.

    The polyphase history of the Los Angeles basin demonstrates the complexity that can occur along active transform margins. Such complexity can be expected in basins that have formed in similar settings.

  3. Page 25
    Abstract

    In the early 18908, Edward L. Doheny, a mining prospector down on his luck, observed residents of Los Angeles gathering ''brea'' from the area's tarpits for use as fuel in coal-scarce California. Realizing that this crude tar was petroleum that had con ealed upon contact with the open air, oheny explored the residential neighborhood near Westlake Park, pooled resources with Charles A. Canfield, an old mining crony, and purchased a city lot for $400. Unaware of oil-drilling methods, Doheny and Canfield began by sinking a four-by-six-foot miner's shaft, digging it out by hand with pick and shoveL They found an oil seep seven feet below the surface and kept digging, despite the presence of gas. They finally gave up at 155 feet, nearlovercome by fumes. Doheny then fashioned a crude drill from a sixty-foot eucalyptus tree trunk and continued to bore the hole. On the forieth day of work, gas burst out of the hole and oil bubbled up into the shaft. The boomwas on.

    With fortunes to be made, the residential district became crowded with promoters, drillers, and derricks. Trampled gardens, chugging and wheezing pumps, flooded lawns, and other nuisances went along with the attempt to tum backyards into pay dirt. In an area bounded by Figueroa, First, Union, and Temple streets, more than 500 wells were producing oil by 1897 (Figure 1).

    Drilling wells in the Los Angeles City field posed the problem of making oil production compatible with urban living. Residents had to deal with noise, dirt, traffic, odors, and waste disposal. At least one solution to the waste disposal problem proved unique. A homeowner with a rig in his backyard had no place for a sump in which to run waste water and mud.

  4. Page 35
    Abstract

    The Los Angeles basin formed in late Neogene time on a continental margin previously shaped by Cretaceous and early Paleogene subduction, Paleogene terrane accretion, and mid-Miocene rifting and block rotation. During Neogene time, the boundary between the Pacific and North American plates shifted progressively eastward beneath the Los Angeles region, creating the broad San Andreas transform zone. As reviewed in this paper, structures and rocks within the Los Angeles basin document each stage of that Neogene evolution.

    The Los Angeles basin began to take its present shape in late Miocene time (ca. 7 Ma) by subsidence between the right-oblique Whittier and Palos Verdes fault zones and the left-oblique Santa Monica fault system. The principal phase of basin opening involved early Pliocene extension in a northwest direction, which accompanied the opening of the Gulf of California and the eastward shift of the southern San Andreas fault to its present position. Most of the structural traps that hold the basin's oil fields began to form during this latest Miocene-early Pliocene deformation.

    Since mid-Pliocene time, many of these traps have been altered and enhanced—and a few have been breached—by Pasadenan deformation, involving southward shortening, the uplift of the Transverse Ranges, and the propagation of blind thrusts beneath the northern Los Angeles basin. The rapid transition from early Pliocene extension to late Pliocene contraction was associated temporally with a change in relative plate motion dated at 3.9- 3.4 Ma. In analyzing Pasadenan deformation, the flake-tectonics model is more appropriate than the fold-and-thrust-belt model, although both models incorporate aseismic detachment at midcrustal depths. The flake-tectonics model is valid for all phases of Neogene deformation, both transtensional and transpressive, in the Los Angeles region.

    Fields discovered to date in the Los Angeles basin will yield an ultimate 10.4 billion oil-equivalent barrels (GOEB) of petroleum. Of this, approximately 73% is trapped in faulted anticlines, 12% in simple anticlines, 10% in fault traps, and 5% in stratigraphic traps. Folding has been controlled primarily by preexisting structural hingelines and sedimentary wedge belts and secondarily by en echelon folding associated with wrench faults. Oil seeps and Quaternary topographic uplifts led to most of the discoveries prior to 1925 along the Whittier and Newport−Inglewood fault zones and in the Coyote Hills. Most later discoveries, including the 3-billion-barrel Wilmington oil field, were in structures with little or no Quaternary expression.

  5. Page 135
    Abstract

    The Los Angeles basin is one of the Neogene basins along the Califomia continental margin that was formed by extension related to complex wrench-fault mechanisms. These extensional mech- anisms caused the pull-apart tectonics that resulted in rapid deepening of many of the continental margin basins in the late Oligocene to early Miocene.

    For the last 60 years, the biostratigraphic framework for correlation between the oil fields of the Los Angeles basin has been based on the benthic foraminiferal zones and divisions first published by Wissler (1943,1958). These biostratigraphic units are used to correlate the upper middle Miocene to upper Pliocene clastic reservoirs across the basin. The benthic foraminiferal zones of Wissler (1943, 1958) were correlated to the Miocene benthic foraminiferal zones of Kleinpell (1938, 1980) and the Pliocene- Pleistocene stages of Natland (1952).

    With the utilization of other microfossil disciplines (e.g., siliceous and calcareous planktonic microfossils), a more refined biostra- tigraphic scheme has been developed. Correlation of plankton biostratigraphies with the radiometric time scale has in tum allowed correlation and calibration of benthic foraminiferal zonations. Application of this new biostratigraphic- chronostratigraphic scheme can now be used to constrain the timing and magnitude of tectonic and depositional events marking basin development.

    Benthic foraminiferal assemblages from three stratigraphic sections, located around the margins of the Los Angeles basin, were correlated to siliceous microfossils, calcareous nannofossils, planktonic foraminifers, and radiometric dates to determine the age relationships of the different benthic foraminiferal zonations.

    Three regional cross sections were constructed based on the thickness of the Neogene sedimentary package and based on the chronostratigraphic relationships between the benthic foraminif- eral zones and the other fossil groups. These regional cross sections illustrate the shifting of the central basin depocenter through the late Neogene. The pre-late Miocene deposits are thickest along the basin margin, especially in the northwestern part of the central block, the Puente Hills, and the Capistrano embayment. The sediments that were deposited during the late Miocene through the early Pleistocene were deposited in the central trough, bounded by the Newport−Inglewood, Santa Monica-Raymond Hill, and the Whittier fault systems.

    Unconformities and hiatuses are associated with each tectonic block. Some of these unconformities are local events related to structural growth or fault movement. The hiatuses are regional events caused by changes in ocean chemistry or velocity of bottom currents.

    The unconformities that occur in the late Miocene to early Pliocene are related to local structural growth in the Palos Verdes Hills area and the Anaheim nose area of the southeastern portion of the basin. Other unconformities that occur in the late Miocene are related to movement along the Newport−Inglewood fault and the Whittier fault.

    In the late middle Miocene a hiatus occurs within the Monterey Formation in the Palos Verdes Hills and the Newport Bay area. This hiatus is thought to be related to climatic and oceanic events associated with continental and oceanic glaciation.

  6. Page 185
    Abstract

    The central Los Angeles basin represents the deepest part of a basin that apparently resulted from rapid and prolonged lithospheric thinning owing to extension between rotating blocks. Subsidence in this tectonic setting began about 18 Ma and presumably reflects isostatic adjustment to the thinning of the buoyant crust. Sediment starving in the period immediately following the initiation of rapid subsidence resulted in a deep water-filled basin that reached water depths in excess of 2 km during Pliocene time. Sedimentation accelerated immediately following the widespread extrusion of andesitic and basaltic volcanics about 16 Ma. Maximum tectonic subsidence, which may require 50% to 75% of lithospheric thinning under the central deep, is about 3 km depending on assumptions. This amount of thinning can be used to estimate the maximum time-temperature history of basin sediments. The pattem of subsidence is best explained by a model of crustal rotation between right-slip faults that results in both extension in the early development of the basin and compression in the later phase of basin development.

  7. Page 197
    Abstract

    The Los Angeles basin is one of the most prolific petroleum- producing provinces in the world. The basin has produced in excess of 6 billion barrels of oil (GBO) and over 7 trillion cubic feet (tcf) of gas and includes one of the worlds largest single accumulations, the Wilmington field. Oil gravities are highly variable, ranging from less than 10° API in shallow producing zones to condensate (>50° API) in a few deep fields. However, much of the oil produced in the basin is rather heavy 25° API) with an appreciable sulfur content(>1%).

    The Los Angeles basin contains abundant organic-rich source rocks containing kerogen rich in sapropelic material. Maturity estimates of the source rocks based on vitrinite reflectance values are low. This appears to be related to the sapropel-rich kerogen, which may generate oil at lower maturities than is conventionally accepted or may cause suppression of vitrinite reflectivity in the kerogen. Maturity estimates based on bitumen production indicate that upper Miocene rocks in deeper parts of the basin are the source of the oil and gas accumulations. Vertical migration of this oil and gas into shallower reservoirs is a feature of many fields, especially the giant Wilmington and Huntington Beach fields.

    Oil quality within the basin varies geographically, with higher quality (high API gravity, low sulfur content) oils in the northeast of the basin and lower quality oils in the west and south of the basin. The variation in oil quality does not appear to be caused by differences in kerogen type, because the sterane biomarker ratios and carbon isotope ratios of the oils are very similar and are consistent with a marine-derived kerogen as the source. Ratios of biomarkers and lower molecular weight hydrocarbons suggest that the superior quality of the oil in the northeast of the basin may be caused by higher maturity, greater migration, a more oxidizing depositional environment, or a combination of all three. Oils appear to be genetically related to the associated gases in a reservoir, except for some cases of localized mixing of gases with microbial methane.

    Depth-related variations in oil quality and in carbon dioxide content in the associated gas appear to be caused by biodegra- dation. A suite of oils and associated gases from the Salt Lake area in the northwestern basin shows that a variety of interrelated physical and chemical changes in many shallow oil and gas deposits results from microbial oxidation of liquid and gaseous hydrocar- bons to carbon dioxide. This is an important process in the giant Wilmington and Huntington Beach fields, where deep reservoirs contain a medium quality oil; whereas shallower reservoirs contain a genetically similar oil that has been microbially transformed to a lower gravity, higher sulfur crude.

  8. Page 221
    Abstract

    Unmetamorphosed strata of Turonian to middle Miocene age exposed around the margins of the Los Angeles basin (LAB) predate the formation of the basin and respond to a different structural framework. Highly organic middle and late Miocene (Luisian and Mohnian) strata also predate the present LAB framework: the Puente Hills received a thicker sequence than did the area southwest of the Whittier fault, and the Santa Monica Mountains were at the distal end of Mohnian turbidites derived from farther north. The central trough became a major depocenter during deposition of the upper Mohnian (after 8 Ma), and it achieved its present northwest-southeast trend about 4 Ma. The central trough filled with “Delmontian” and Repettian turbidites shallowing upsection to Pliocene and Pleistocene deposits. Miocene and early Pliocene source rocks were buried beneath the oil-generating thermal threshold so that oil migrated to stratigraphic and broad structural traps formed during deposition. Many oil fields contain a thick stack of reservoir turbidites; the boundary between highly productive turbidites and overlying water-bearing turbidites, also in trapping position, is abrupt. Late Quaternary deformation after basin filling distorted rather than enhanced oil traps, and some oil accumulations were breached by erosion. New oil prospects could result from a better understanding of (1) the Mohnian basin framework in contrast to the far different post- Mohnian framework and (2) the post-Mohnian fold and thrust belt tectonicaUy loaded by the southern margin of the Transverse Ranges onshore and offshore.

  9. Page 239
    Abstract

    The Los Angeles basin is a small, deep Neogene basin located in the northeast portion of the southern California continental borderland. It was formed along a transform margin during early to middle Miocene time, as were numerous basins within the continental borderland south of the Transverse Ranges.

    A series of submarine fans were deposited in the Los Angeles basin during the middle to late Miocene, Pliocene, and Pleistocene.

    The configuration of these fans appears to fit several basin-floor fan models, but the fan morphologies were greatly influenced by local paleobathymetry. The primary sediment transport mecha- nism was turbidity flows from submarine canyons, but other mass sediment transport mechanisms, such as debris flows, fluidized sediment flows, and grain flows, were also significant.

    Three primary, commonly coalescing, submarine fans have been recognized: the Tarzana fan in the northwestern Los Angeles basin, the San Gabriel fan in the north central portion, and the Santa Ana fan in the eastem portion of the basin. Most of the oil produced in the Los Angeles basin comes from upper Mohnian, Delmontian, and Repettian sandstone and conglomerate reservoirs of these submarine fans.

    The Los Angeles basin was a probable silled basin that intersected the oxygen-minimum oceanographic zone during the late Miocene and Pliocene. A combination of the rich biogenic sedimentation along with rapid burial by coarse- to fine-grained clastics and moderate paleo-heat flow provided almost perfect conditions for the generation and migration of oil and gas. Stmctural deformation was intermittent throughout the Neogene but reached a culmination in the late Pleistocene to Recent.

  10. Page 261
    Abstract

    The Taranaki basin is a Cretaceous and Tertiary sedimentary basin located along the western side of the North Island, New Zealand. Initiated during the Cretaceous, the Taranaki basin lies at the southem end of a rift that developed subparallel to the Tasman Sea rift, which now separates Australia and New Zealand.

    Structure of the basin has been controlled by movement along the Taranaki and Cape Egmont fault zones. Subsidence commenced in the Cretaceous and continued until the Pliocene. The predominant tectonic regime in the Taranaki basin changed from one of extension to one of compression in the early Miocene. Late Tertiary tectonics formed three primary structural types: faulted anticlines, high-angle overthrust structures, and tilted fault blocks. Kapuni and Maui gas-condensate fields are faulted anticlines; the McKee (oil) and the Ahuroa and Tariki (gas-condensate) fields are overthrust structures.

    Sandstones within the Eocene Kapuni Group and the Oligocene Otaraoa Formation are the only producing reservoirs. The gas- condensate and oil are sourced from nonmarine to paralic coals and carbonaceous shales of the Late Cretaceous-Eocene Pakawau and Kapuni groups. The overlying marine sequences are organically lean and have negligible source potential.

    A large proportion of the sedimentary succession was deposited during the late Tertiary. Consequently, the geothermal gradient in the Taranaki basin is moderately low «3°C/100 m). Maturation studies show that only those source rocks buried between 4000 and 4950 m are in the present oil generation and gas expulsion window.

  11. Page 283
    Abstract

    The Magdalena River flows northward across the Colombian Andes traversing a series of en echelon, sediment-filled structural de- pressions. The Magdalena basins resist easy classification in that, until the late Miocene, they have been parts of much more extensive basins: an extensional, back-arc basin during the Triassic-Jurassic; a pericratonic trough during the Cretaceous and early Tertiary; the inner margin of a broad, east-facing foreland trough during the mid-Tertiary; and more recently an array of intermontaine or “successor” basins. The geologic character of the Magdalena basins is tied intimately to that of the bordering Central and Eastern Cordilleras. Since 1918, there has been nearly continuous exploration activity in the Magdalena basins resulting in the discovery of more than 2.6 billion barrels of oil (GBO) and 2.7 trillion cubic feet (tcf) of gas—more than half of the total oil and about a third of the total gas reserves of the country. As of the end of 1989, the daily production from the basins averaged 143,432 bbl of oil and 182.8 mcf of gas.

    The abundant hydrocarbon resources of the Magdalena basins are based on the presence of a thick, organic-rich limestone and shale succession (La Luna or Villeta) deposited in an extensive pericratonic trough along the northwest margin of the Guyana shield during the Cretaceous. In the south, nearer the paleogeo- graphic margin of the trough, shallow-marine sandstones (Caballos and Monserrate) bounding the Cretaceous marine megacycle are the prime reservoirs. To the north, nearer the axis of the trough, Cretaceous sandstone reservoirs are absent and production is almost exclusively from mid-Tertiary molasse deposits. The Magdalena basins contain a wide variety of structural and stratigraphic traps, most developed during or prior to peak of maturation of the Cretaceous source beds. Recent discoveries of

    giant oil accumulations, such as the San Francisco field, were made in large, hanging-wall anticlines previously considered breached and unproductive. The testing of deeper reservoirs and new structural concepts during the 1980s have resulted in many important discoveries. From the standpoint of hydrocarbon exploration and exploitation, the Magdalena basins are not yet “mature.” The potential for additional major discoveries is excellent and certaini. i. that with improved production techniques current estimates of remaining ultimately recoverable reserves in the producing fields will be revised upward.

  12. Page 303
    Abstract

    The Falcon basin, located in northwestem Venezuela, has been intermittently explored since 1912. Since 1912, 200 exploratory wells have been drilled and 12,000 km of seismic lines have been acquired. This exploration effort has resulted in the discovery of eight small producing fields in both onshore and offshore areas.

    The geologic history of the basin began in the late Eocene, and deposition continued through the Pliocene to the Recent. Because the basin is located at the boundary between the Caribbean and South American plates, sedimentation was controlled primarily by tectonism as evidenced by seismic and well data.

    Three structural systems developed as a result of east-west dextral crustal movement. The first, consisting of a set of normal faults and associated horsts and grabens, forms the northern extension of the Oligocene-Miocene basin. The second system, known as the Falcon anticlinorium, includes east-northeast- striking parallel folds located in the center of the basin. The third structural system encompasses the active east-striking right-lateral strike-slip faults of which the Oca fault is the most relevant, owing to its regional extent.

    The stratigraphic discontinuity within the basin is one of its principal features. The two stratigraphic stages that have been recognized are the result of a late Eocene to early Miocene transgression and a middle Miocene to Recent regression.

    The northern flank of the basin, including the offshore area, has generated hydrocarbons from Oligocene and lower Miocene marine source rocks. However, small quantities of crude oils of terrigenous origin have been generated from Eocene source rocks.

    Based on the tectonic and stratigraphic framework of the Falcon basin, a new conceptual model is proposed that can be applied to future hydrocarbon exploration in the area.

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