Technical Meeting and Luncheon: Tuesday, February 10th, 2015

Where: The Petroleum Club of Lafayette

When: 11:30am

Lunch: $20.00 for members ** Be Sure to Bring Your 2014-2015 Membership Dues!**

Luncheon Talks:  

Detecting hydrocarbon expulsion from source rock in seismic data: case studies from offshore, Namibia and Gulf of Mexico shelf

By: David Connolly, dGB Earth Sciences

Abstract:

Many Tertiary, Mesozoic, and Paleozoic petroleum systems are dominated by vertical hydrocarbon migration from deep thermally mature source rocks. On seismic data the vertical migration paths are generally recognized as vertically aligned zones of chaotic often low amplitude reflectivity. This seismic character is due to two major causes: 1) scattering of the seismic signal due to low saturation gas in the sediment; 2) fracturing, related to vertical migration. This vertical hydrocarbon migration is variously described as gas chimneys or gas clouds. However, in many basins it can be very widespread, and originate from known or suspected source rocks. Evidence of this vertical hydrocarbon migration is often overlooked in the seismic record for a number of reasons. One, hydrocarbon migration often occurs in the deep over- pressured interval below major producing reservoirs. Two, hydrocarbon migration often occurs in shale prone intervals which are poorly imaged due to low acoustic impedance contrasts. Three, migration is often focused on structural highs related to diapiric salt or shale, which is also poorly imaged. Lastly, hydrocarbon migration largely occurs in intervals with little well penetration to confirm their presence.

Because of the diffuse nature of these hydrocarbon migration pathways, they are difficult to map in three dimensions. To address this issue, a method has been developed to detect these petroleum migration pathways in 3D seismic data, to map their distribution and to allow them to be visualized in three dimensions. The method involves an interpreter picking examples of obvious vertical hydrocarbon migration (gas chimneys) in normally processed seismic data. A set of seismic attributes is chosen, which highlight the feature. Then a neural network is applied to the seismic volume to create a “chimney probability volume” to highlight possible hydrocarbon migration pathways, which are similar to the picked examples in seismic character. Not all vertically aligned, low amplitude, chaotic seismic reflectors represent hydrocarbon migration. Therefore, the subjective selection of training locations and the resulting neural network predictions are validated by objective criteria before the results are used in geological applications. Once we are able to visualize these chimneys in either 2D or 3D seismic data, we can then potentially determine from which interval or intervals, they originate (potential source rocks). Which source rock is providing charge to a reservoir can have important implications for determining whether the reservoir will produce oil or gas.

Case studies will be shown from offshore, Namibia, where hydrocarbons expelled from a Cenomanian oil prone source rock may be providing hydrocarbon charge to Upper Cretaceous canyon head reservoirs. We will also show an example from the Texas Gulf of Mexico shelf, where we can demonstrate expulsion from the Eocene gas-prone source rock and vertical migration into shallow Miocene gas bearing reservoirs. We will also show a case study from Grand Bay Field where chimney results indicate reservoirs are being charged by both Eocene gas-prone source and deeper Jurassic oil- prone source. Additional examples from offshore, Louisiana will also be shown.

 

Speaker Bio

David Connolly is dGB Earth Science’s Chief Geologist. He has over 30 years of industry experience in various aspects of petroleum geology and geophysics. He began his career with Analysts / Schlumberger in Core Analysis in Lafayette. He worked for Getty / Texaco from 1981-2001 as a Petroleum Geoscientist in a variety of international and national exploration assignments. He has been with dGB since 2001. He is co-editor for the SEG/AAPG Geophysical Developments #16, Hydrocarbon Seepage: From Source to Surface. He has been one of the main architects of the “Chimney Catalogue” which documents the relationship between chimneys and seal/charge risk for over 150 fields and dry holes. His paper Detecting hydrocarbon expulsion from source rock in seismic data; Case studies from offshore, Namibia and Gulf of Mexico shelf, presented at the 2014 SEG Annual Meeting, was honored as a top paper. He graduated from Washington & Lee University with a B. A. in Geology.

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