The Ventura Basin is located in southern California, just north of Los Angeles and on the western side of the Transverse Ranges. It is approximately 215 miles long and 30 miles wide, with an estimated sedimentary volume of 40,000 cubic miles. The Ventura Basin is bounded by the Red Mountain reverse fault and the San Cayetano reverse fault to the north. The Oak Ridge Fault is the boundary to the south. The basin also includes the Northern Channel Islands and the water that surrounds it. The Ventura Basin can be further divided and analyzed as east and west divisions of the basin. The western part of the Ventura Basin has an offshore extension that is known as the modern Santa Barbara Basin. The Ventura Basin is the deepest basin from the Cenozoic time in California, and contains more than 58,000 feet of marine sediments. The location of the Ventura Oil Field in southern California can be seen in Figure 1.
The Ventura Basin is an east-west trending sedimentary basin that was formed in the Miocene time, during the Tertiary Period of the Cenozoic era. The estimated time frame of formation is between 4 million to 22 million years ago. The basin was created as a response to the shifting of tectonic plates of the San Andreas fault. The Ventura Basin was the location of a forearc basin of the East Pacific Rise in the Pacific Ocean. Depositions of sediments of Cretaceous and early Tertiary until the East Pacific Rise were subducted. Roughly 22 million years ago during the early stages of the Miocene time, stretching of the crust formed the Ventura Basin. This allowed for Miocene deposition to occur. Nearly 6 million years ago, the basin got deeper and Pleistocene deposition began to accumulate. During the time of the middle Pleistocene, the area was subject to faulting, uplift, and folding. The basin was nearly 5,000 feet under water during this time.
The western part of the Ventura Basin holds the largest oil fields because of the large structures and presence of Monterey formation source rock and Modelo source rock. Oil and gas production began in the Ventura Basin in 1861 due to the discovery of the Santa Paula oil field. As a result, other oil fields were discovered in the area. This made Ventura County one of the major producing areas in all of California. It is estimated that there is around 3,800 active wells in Ventura County today. In Ventura county, Aera Energy is currently the largest onshore oil producer, and began production there in the 1920s. Their operations span across 4,300 acres, and produce on average 12,000 barrels a day of crude oil and 7 mmcf of natural gas a day.
A seal is a rock that forms a barrier around a reservoir and prevents the oil and gas from migrating out of the reservoir. The seals that prevent the oil and gas from migrating in the Ventura Basin are shale units from the Paleogene and Neogene sequences.
Source Rock & Migration
During the Pliocene-Miocene time, the basin was simply structured with wide open space, which allowed the hydrocarbons to move easily. Hydrocarbons formed from marine organism depositions by the sea of fine grained sediments, resulting in migration into different traps. The source rock is the Monterey-Modelo formation from the Miocene. The Monterey formation is composed of shale and limestone. The permeability of shale is not ideal compared to that of sandstone. Limestone is more permeable, making it easier for the migration of hydrocarbons.
Most of the traps in the Ventura Basin are anticlines. Figure 2 shows a diagram of an anticline trap with hydrocarbons trapped below the surface. These traps have formed from the Pliocene time to recent times. There are some stratigraphic traps due to thrust faults that were created in the last five million years as a result of faulting and folding. In the eastern part of the basin, growing anticlines had trapped the oil that had already been generated. In the western part, anticlines were not as full, so the presence of stratigraphic traps were able to trap the hydrocarbons in place.
The Ventura Basin is comprised of multiple stacked formations. Most of the reservoirs are sandstones, and they have porosities and permeabilities that are favorable for oil and gas production. The reservoir rock is the sandstone of the Repetto formation, also known as the lower Pico, and it underlies the upper Pico formation from the Pliocene time. The Pico formation can be analyzed by the qualities of the upper and lower sections of the formation. The lower Pico formation is about 9,300 feet thick and is composed of sandstones, clay, and heavy conglomerates, while the upper Pico is composed of silty clay. The reservoir rock must be porous and permeable to allow for the oil and gas to migrate to from an area of high pressure to an area of low pressure.
Risks & Uncertainties
The Ventura Basin is located along active reverse faults, as shown in Figure 3. This basin is one of the more active tectonic plate regions of the world and is considered a significant seismic hazard to the urban population. Hydraulic fracturing near fault lines could lead to greater risk of earthquakes. However, 3D seismic imaging can also be used to identify potential areas of movement and minimize or avoid the risk of earthquakes. Additionally, the wildfires that have plagued California in recent years presents another risk for production. Due to the droughts that California often experiences, the conditions for production are not favorable considering there is not much water at an operator's disposal for drilling. Legislators and activists argue that oil and gas recovery is a leading factor of climate change and that it is one of the main factors for the spike in wildfires.
Risk of Government Policy
In recent years, California has been known for opposing fracking. Earlier this year, California's governor, Gavin Newsom, signed an executive order that would effectively ban all hydraulic fracturing in the state of California by the end of 2024. Shortly thereafter, Governor Newsom approved the permits for six new wells. These wells are owned by a company that he has lobbying ties to. Legislators are pushing hard for new laws to ban all types of oil drilling. This comes at a time when wildfires caused by climate change reached record highs. The uncertainty and inconsistency of Governor Newsom's actions, paired with natural disasters, makes pursuing new target areas a big risk.
Petroleum & Facility Engineering
There are currently 43 companies that are operating facilities throughout the 34 active fields that are located in Ventura County. Meanwhile, there are only two major oil and gas processing facilities onshore that process oil and gas from offshore production. Each facility includes local pipelines that can move production fluids from the wellhead to the storage and processing facilities. From there, they transport that oil and gas to the major pipelines to the regional refineries that are located near Los Angeles.
Currently, one of those operators with oil and gas activity is the California Resources Corporation. They are currently developing and operating 27 fields across the Ventura Basin. These fields are being developed using conventional recovery methods, as well as unconventional methods and enhanced oil recovery methods.
Future Petroleum Potential
The Ventura Basin has been explored and developed since 1861. There has not been a large conventional discovery onshore in the last several decades, and it is believed that it will remain that way. As of 2018, there are proved reserves of only 31.5 million barrels of oil equivalent. The volumes of undiscovered continuous oil located in the deep basin is expected to be small because of the large volume of oil that has migrated to more shallow, conventional traps. This is an indicator that large volumes of oil are not being retained in the source rock.
However, the Ventura Basin in California is regarded by some companies as an area that presents an opportunity for companies to expand their asset portfolio of previously drilled, conventional light oil producing wells that have low operating costs. As mentioned before, Aera is the largest producer in Ventura County. Their wells in the area are now in the secondary recovery phase. This involves injecting water into the reservoir to force out an remaining oil and bring it to the surface. When the reservoirs get to the secondary recovery methods, it generally means that they are on their last legs until they are completely depleted. However, The use of 3D seismic has allowed for greater recovery volumes and success rates compared to wells drilled without 3D seismic. Figure 4 shows the onshore 3D seismic process for discovering potential oil fields. This could be used to locate previously developed wells that may still have hydrocarbons that can be recovered. The California Resources Corporation believes that the multi-stacked formations provide an opportunity to revisit and explore these previously developed oil fields. They believe that these previously developed fields could continue to produce oil and gas, as well as provide insight on potential new plays.
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