Wednesday, May 16, 2018

AAPG Annual Meeting 2018

Presentations (speaker underlined):

Monday, 10:50-11:10 am, Ballroom A
Seismic Characterization of Natural Fractures in the Buda Limestone of Zavala County, Texas 
A. Smirnov and C.L. Liner

The Buda Limestone is a naturally fractured Early Cretaceous carbonate formation in south Texas which is unconformably overlain by the Eagle Ford Shale. Matrix porosity of the Buda is less than 6%, therefore natural fractures improve the potential for commercial hydrocarbon production from this tight limestone formation. It is a challenge for producers to identify these zones using well log and poststack 3D seismic data which typically available to medium or small exploration companies. This project provides a workflow based on well log analysis tied to seismic acoustic impedance (AI) inversion to locate areas of probable natural fractures.

Acoustic impedance inversion was performed across a 40 square mile 3D seismic survey. The AI data shows low impedance shadow zones on the down thrown side of faults. Post stack geometric seismic attributes such as coherence, maximum and minimum curvature were analyzed in the anomalous AI areas, along with physical seismic attributes such as RMS amplitude and instantaneous frequency.

To map primary porosity, a relationship between acoustic impedance and porosity is established by crossplotting well log data. A linear fit to the Buda data in one well indicates a robust correlation between sonic porosity, density-porosity and AI. Sonic porosity is an indicator of the matrix porosity in the Buda Limestone, while density porosity represents both matrix and fracture porosity. Using the trend line equation for AI vs sonic porosity, the 3D seismic impedance volume was scaled to a matrix porosity volume.

In the downthrown faulted areas, the porosity volume indicates values greater than expected of matrix porosity. This has been reported elsewhere in carbonate reservoirs as an indicator of enhanced secondary (fracture) porosity. This study indicates that a combination of acoustic impedance inversion and seismic attributes can identify areas of enhanced natural fracturing within the Buda Limestone interval.

Tuesday, 9am, Exhibit Hall
Mapping Lower Austin Chalk Primary and Secondary Porosity Using Modern 3-D Seismic and Well Log Methods in Zavala County, Texas [poster 77]
D. Kilcoyne and C.L. Liner

Establishing fracture distribution and porosity trends is key to successful well design. The Austin Chalk has historically been referred to as an unpredictable producer due to high fracture concentration and lateral variation in stratigraphy, however recent drilling activity targeting the lower Austin Chalk has been very successful. The Upper Cretaceous Austin Chalk (AC) and Eagle Ford (EF) units are considered by many to act as a single hydrocarbon system so both units are investigated. Communication between these two units is largely through expulsion or dewatering fractures, extensional faults or along the AC/EF unconformity. Total porosity for the Eagle Ford is composed of a primary matrix component and secondary fracture porosity. For the Austin Chalk, the secondary porosity includes both dissolution and fracture components which complicate wireline and seismic interpretation.

The current study interprets 40 square miles of modern 3D seismic data for horizons and faults using amplitude, coherence and ant tracking seismic attributes. Post stack acoustic impedance (AI) inversion is applied to the time migrated seismic volume with control from two wells; this input data is similar to that available to independent operators active in the area. Wireline acoustic impedance plotted against density-porosity reveal strong correlations that allow calibration of seismic AI into primary, secondary and total porosity from which time slices and surface maps are created. Relationships are identified between porosity and geological features of interest, such as faulted and brittle zones, that may prove useful in guiding future well development in the lower Austin Chalk.

Wednesday, 11:10-11:30 am, Ballroom C
Tracks, Outrunner Blocks, and Barrier Scours: 3-D Seismic Interpretation of a Mass Transport Deposit in the Deepwater Taranaki Basin of New Zealand 
F.J. Rusconi, T.A. McGilvery and C.L. Liner

A series of Plio-Pleistocene mass transport deposits (MTD) have been identified in the deepwater Taranaki Basin, in New Zealand, using the Romney 3D seismic survey. One of these MTDs has been chosen for description and interpretation based on high confidence mapping of its boundary surfaces. The deposit exhibits an array of interesting features similar to those documented by researchers elsewhere plus a unique basal feature unlike those previously observed. The basal shear surface exhibits erosional features such as grooves, “monkey fingers”, and glide tracks. We have been able to image outrunner blocks at the end of the glide tracks in distal areas of the deposit. 

Internally, the MTD is typically characterized by low impedance, chaotic, semi-transparent reflectors surrounding isolated coherent packages of seismic facies interpreted as intact blocks rafted within the mass transport complex. These transported blocks scale up to 1 km wide and 200 m high, and commonly protrude above the upper surface of the flow. This yields a very irregular paleo-bathymetric surface on the top of this and other MTDs with local relief attributed these protrusions ranging from 10 m to >100 m . The complexity of this upper surface had local impact on subsequent flows. 

The term “shield block” refers to those large protruding obstacles on the paleo-seafloor that acted as barriers to subsequent flows as they advanced downslope. Obstacles such as mud volcanos have been documented to act as such barriers resulting in elongate, downflow erosional remnants as positive features. The opposite is the case for shield blocks, which disrupt flow and result in elongate, downflow erosional troughs that are negative features. These local erosional features are then infilled similar to mega flutes and are preserved as elongate isochron thicks on the downflow end of the underlying shield block. Kinematic evidence provided by various structures suggests that the MTD flow direction was SE-NW toward bathyal depths. The features presented and the absence of extensional headwall structures, such as local arcuate glide planes and rotated slide blocks, suggests that this part of the deposit belongs to the translational to distal domain of the MTD, and its source area is expected to be somewhere toward the SE in a paleo continental slope.

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