California’s Ventura basin has been in production since 1861, the year Lincoln was sworn in as President and the first shots of the American Civil War were fired. Approximately 100 oil and gas fields have been discovered since, with cumulative production from the basin exceeding 4 Bboe.

Although activity has slowed in recent decades, several E&P operators remain intrigued by the basin’s exploration potential. They believe discoveries remain to be made and are working to identify bypassed targets masked by Ventura’s complex geology; deeper zones analogous to Oxy’s 2009 discovery in the nearby San Joaquin basin; and sweet spots in the Monterey shale, the pervasive source rock that has recently become an unconventional reservoir target.

In this gravity model of a portion of the Ventura basin, California, deeper sedimentary depocenters are shown in purple. Producing well locations appear as black dots, dry holes as blue dots. (Images courtesy of NEOS GeoSolutions)

Integrated study

Because of challenging topography, urbanization, environmental concerns, and subsurface complexity, ground-based seismic acquisition is not a practical alternative in most parts of the basin. In 2010, Houston-based NEOS GeoSolutions approached several E&P operators about sponsoring a basin-scale survey that would rely not on any single geophysical dataset but on a methodology in which all available geological, geophysical, geochemical, and petrophysical data would be accessed and simultaneously integrated and interpreted.

A seismic image, when it exists, can be extremely useful in revealing the structures within the earth, but other geological and geophysical measurements can bring even more clarity to the interpretation as they reveal important things about basin architecture, fault and fracture systems, rock properties, and fluid distributions.

To complement existing 2-D seismic lines, well logs, and geologic maps in the public domain, NEOS acquired a series of airborne geophysical measurements over roughly 2,600 sq km (1,000 sq miles) of the onshore Ventura basin. The newly acquired datasets included:

Gravity – to delineate deep basin architecture and

basin-scale structural features;

Magnetics – to map regional fault and fracture networks; and

A NEOS geoscientist “ground-truths” Ventura airborne hyperspectral measurements using a handheld analytical spectral device. An oil seep exists about 2 m (6 ft) behind the geoscientist.

Hyperspectral data – to detect oil seeps and micro-seepage impacts on surface vegetation. On some basin-scale projects like this one, additional datasets such as radiometrics and electromagnetics also are acquired. These nonseismic datasets delivered new insights to the program’s underwriters, even when interpreted individually. For instance, the gravity data showed the presence of a much deeper sediment column in portions of the basin that had limited well penetrations. In addition, gravity measurements highlighted several unknown and untested mini-basins along the edges of the main basin depocenter. An analysis of the magnetic data provided further exploration insight. The vast majority of the fields in the Ventura basin are aligned with identifiable magnetic anomalies that correlate with deep-seated fault systems, something that could be expected in a tectonically active, structurally driven basin. An analysis of the hyperspectral data highlighted a larger number of seeps throughout the basin. Geoscientists on the ground confirmed the seep anomalies detected using the airborne sensor platform. Armed with the location of new depocenters and mini- basins from gravity data, the ability to identify regional fault systems using magnetic data, and the presence of direct and indirect hydrocarbon indicators on the surface from hyperspectral imaging, explorationists are now able to qualitatively identify new leads and play types throughout the Ventura basin.

New exploration tools

The NEOS methodology provides interpreters with an additional tool to aid in the search for hydrocarbons. This is based on a geostatistically driven, software-enabled search for the unique measurements and attributes that correlate with known fields (or high production rate wells) in a basin or designated area of investigation. Once these “correlative anomalies” with known fields (or wells) are identified, a proprietary software package based on pattern recognition algorithms is used to identify the same set of correlative anomalies in areas without well control. In essence, the software is searching for unexploited parts of the basin that share the same set of “geo-anomalies” as the area’s known fields and highest producing wells.

What measurements are used as part of the search? In the case of Ventura, nearly 50 raw datasets and calculated attributes were considered, including items like Bouger gravity and the first vertical derivative of reduced-to-pole magnetic data. The methodology determines the statistical relevance of each measurement and attribute, eliminates measurements and attributes that are not relevant, and mathematically determines the weighting factors to apply to each statistically relevant dataset and attribute. The result is an objective, mathematically driven map of the entire basin, highlighting areas that are being flagged as relatively more prospective or productive. As with any measurement or tool, the interpreter does not blindly follow the output but instead uses the insights provided to derisk previous exploration concepts and to identify possible new leads worthy of further study.

In this geostatistical assessment of the probability of liquid hydrocarbons in the shallow Pico formation, hot colors correspond to a high probability of oil. Black dots represent producing wells within the Pico horizon.

In the case of the Ventura basin, geostatistical methodology was applied to four separate geologic horizons that are producing today, including the shallow Pico formation, the Sespe, the Miocene (effectively the Monterey shale zone), and the deep Eocene. The shallow zones had more well penetrations and discoveries than the deeper intervals. Nonetheless, even the shallow Pico contains several high-potential exploration anomalies that have yet to be drilled. The results become even more interesting as the investigation goes deeper, where the number of well penetrations into geostatistically identified anomalies is even smaller and, therefore, the corresponding exploration potential is even higher.

The methodology is injecting new life into an old basin, arming Ventura operators with the additional data and insights they require to unlock a new wave of exploration in the years ahead. Whether a play is old or new, conventional or unconventional, a multimeasurement interpretation approach can often complement data that already exist and help geoscientists understand the attributes that determine where fields might be located within a basin or why some wells are more productive than others, even when completed similarly within the same geologic horizon or area of the play.

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