The Los Angeles basin is only 30 by 50 km bound on the north by the Santa Monica Mountains, where the outcrop photos were taken, and Puente Hills, and on the east and south by the Santa Ana Mountains and San Joaquin Hills. The Palos Verdes Peninsula marks the outer edge of the basin along the coast. The basin covers an area less than 1400 km2 (the equivalent of 2 North Sea blocks). Basement is over 9 km deep with total basin relief exceeding 13 km. Sixty-two oil fields have been discovered. Fourteen major fields (>100 MMbo, 16 MMm3) were discovered between 1920 and 1980 and they are all still producing. Three of the major producers account for over half of the 8+ Bbo (1.3 Bm3) extracted so far in the basin. The 3 Bbo (480 MMm3) giant Wilmington oil field is the largest.
The Los Angeles basin has a long history of oil use and exploration. Oil from seeps was utilized by Native Americans for medical and sealing purposes. In the mid 1800’s oil was collected from pits and seeps to distill as lamp oil. Through the late 18th century, most of the oil activity, including discoveries, was associated with the surface oil seeps and included the discovery of the Los Angeles City field in 1892 (Geo ExPro, v.4, no. 2).
This discovery touched off a basin wide oil search. By 1900, over 1,000 wells were drilled leading to the first discoveries of oil fields not associated with surface oil seepage.
These early 1900s discoveries were the first to teach geologists that local structural hills were indicators of subsurface oil reservoirs.
Complex Beginnings
The picture above shows Dr. Eugene Fritsche, California State University, Northridge, pointing out sedimentary structures that indicate turbidite deposition exposed in the Santa Monica Mountains. This Santa Monica Mountains outcrop is only about 12 km northwest of producing fields in the basin. Photo: Tom Smith.The Los Angeles basin was created in a script that even Stephen Spielberg would find difficult to follow.
Dr. A. Eugene Fritsche, from California State University Northridge, gives one of the more credible, as well as colorful, renditions. His website (Geology Trips, Evidence of the Incredible Miocene Rotation, 2001) provides a good understanding of the complex interaction and rotation of plates leading up to basin formation.
As the basin floor opened up, rapid deposition of shallow-marine to deep-marine sediments started to fill in a new basin consisting of both potential hydrocarbon source and reservoir rocks. Sand derived from the recycling of older units was eroded from nearby uplifts and then deposited as potential reservoir units. Highly organic-rich shale beds of the Monterey (Modelo) Formation interfingered with these reservoir rocks.
Later, northward compression of the basin against rock units to the north, along with shearing associated with the proto Newport-Inglewood fault (a northwest trending, right-lateral, wrench fault system), resulted in the rocks within the northern portion of the basin being highly deformed. Those rocks on the northernmost edge of the basin were exposed during uplift of the Santa Monica Mountains, while faulted anticlines formed along the wrench fault systems crossing the basin.
The rapid burial of a good source rock and a high geothermal gradient created an excellent hydrocarbon generation window that supplied the many recently formed structural traps.
Complex Deposition
The dominant reservoirs in the Los Angeles basin are Miocene to Pliocene stacked turbidite sandstones derived from a complex of coalescing submarine fans. The fans were sourced from the north and northeast filling smaller sub-basins created by concurrent normal faulting and rapid subsidence. Water depths were in the upper to middle bathyal range with abrupt facies changes occurring from course-grained sandstone to shales in the lower depositional areas.
Some of the fans were in large, well-developed systems that possess a channelized inner fan that fed course materials onto the abyssal plain. Even in these large fan systems, changes in sedimentation can be laterally abrupt. For example, the distance between the Long Beach and Wilmington fields is only 5 km. The 2 fields contain the same 7 producing zones, however the Long Beach field, being to the east and more proximal, has over 2,300 m of oil column in a much thicker sandstone section. Stratigraphic changes field-wide have prompted division of the 7 productive zones in the Wilmington field into 52 subzones resulting in better reservoir modeling.
Deposition in the smaller fans is an even bigger challenge for the geoscientist trying to model the reservoirs. The present day California coastline south of the Santa Monica Mountains has a relatively narrow shelf and provides a possible depositional model, while outcrops help piece the picture together.
The small fans started when sand on the shelf edge accumulated, became unstable and then spilled off the shelf in the form of small turbidity currents. Over time, this activity eroded a small submarine canyon in the shelf. The canyon gradually extended landward, incorporating coarser grained sediments that consequently increased the erosive power of the turbidity currents. The steeper canyon sides carved into fine-grained slope deposits, became unstable, and began to slide down the canyon in the form of slumps and debris flows that accumulated on top of the turbidity-current deposits. The cycle ended, possibly due to sea-level rise or onshore river channel movement, and this small fan deposit became covered with slope and mud deposits. Such fan deposits could form substantial depositional traps.
Complex Structure
This section examplifies the complex structures that are encountered in the subsurface of the Los Angeles Basin.Three active wrench zones, the right-lateral Palos Verdes, Newport-Inglewood, and Whittier faults, cross the Los Angeles basin from southeast to northwest. These 3 fault systems are responsible for forming adjacent en echelon complex folds that trap the basin’s oil.
Major stratigraphic traps are not common but do occur within individual fields. However, exploration of the basin using modern seismic techniques that could find more subtle traps has not happened here because of the urban environment. Exploration and drilling is now restricted to existing fields.
Not only is the structure of the basin rather complex, individual fields tend to be quite complex and asymmetrical with one limb near vertical to overturned. Cross faulting further complicates these structures. One example is the Sawtelle field. The original discovery pool is only 200 feet wide in an anticline with limbs near vertical. A second pool is a subthrust play on the footwall and has units that are stretched and overturned.
Seismic is poor over most of these areas and of little use. Good structural modeling is accomplished with the use of dipmeter and other log information and a lot of redrills.
