1. Existing Subsurface Model
Processing Of A High-Frequency Surface-Sourced 2D Seismic Line Shot Over The
Fluvial Wilcox Play In LaSalle Parish, LA
Zachary Ghalayini1, Rui Zhang1, Gary Kinsland1, Robert Stewart3, and Doug Temple2
1. University of Louisiana at Lafayette 2. United Service Alliance 3. Applied Geophysical Lab, University of Houston
If a shallow seismic survey can help differentiate between sand and shale channels of the Fluvial Wilcox play, then the benefits would outweigh the costs of another dry hole. Previous geologic studies in this basin are extensive, and are created primarily by legacy production and well log control. The strata of the upper Wilcox Group has never been imaged with sufficient resolution in seismic data to be of much use. The low-resolution images too often produced are essentially due to the low-frequency spectrum used to target the deeper play trends. We wanted to test if high-frequency surface sourced data could image these shallow plays better than traditional seismic data. The advantage of using a high-frequency source is that the shallow horizons, ranging from 1500 to 3000 feet in depth of the Tullos-Urania oilfield, have become amplified with higher resolution. Constructing and interpreting the seismic cross sections was carried out by incorporating a full processing sequence including noise suppression, velocity modeling, stack imaging, and migration. The results of this research have advanced the accuracy of the current geologic structural interpretation of the Tullos-Urania oilfield. In the future, operators who may not have the funds or desires to target deeper plays can use this method to aid in exploration and development and drive down costs associated with uncertainty.
Abstract
Existing Subsurface Model
Geologic Structure of the Tullos-Urania field, showing contours on top of the producing sand (marked by the star on the Stratigraphic column) with CI: 10“. The red line in the figure illustrates the location of the 2D seismic Line. The seismic survey was shot along dip to portray structural variation best. The stratigraphic column displays the major horizons of the petroleum system along with the oil producing zones (Highlighted in Green). (Schneider, G. W., 1929)
Data Acquisition
Noise Suppression
Kirchhoff Post-Stack Time Migration
Kirchhoff Post Stack Migration
The left cross section was run on the constant velocity stack at 2000 m/s, the figure on the right illustrate a migrated stack using the dynamically picked velocities.
The source was a Nitrogen-accelerated, truck mounted, 200lb weight drop mechanism (United Service Alliance Model A-200). The receivers were vertical component geophones laid out at 5-meter intervals along the 2D line.
Signal Processing using noise suppression, the figure above illustrates (from left to right) the raw binned gather, post-Linear Noise attenuation, post-Despike, and post Frequency Domain Noise Attenuation. LNA suppresses linear energy within a defined velocity range which aids in the ground roll and first break muting. Despike removes strong energy based on a median threshold defined in Freq-space domain. FDNA suppresses noise bursts, spikes, and other noises.
Processing and Imaging workflow
Discussion
Signal Processing – Improving signal to noise ratio with several noise
suppression modules
Stack Imaging - Stacking the Inline with a constant 2000 m/s velocity and also with a dynamically picked velocity. The velocity analysis uses a combination of semblance analysis and constant velocity mini stack panels.
Applying Kirchhoff Post-Stack Time Migrations on both stacked lines and comparing the results
Applying Kirchhoff Pre-Stack Time Migrations
Raw Data
Cmp Sorting and muting
Assign Geometry and apply refraction statics
The aspiration of this research was to process two-dimensional reflection seismic data collected along the dip of the Tullos-Urania Oilfield and to analyze the economic feasibility of such a survey in the future. Processing and imaging this data was a demanding task because of the significantly low signal-noise ratio.
Processing was accomplished mainly using the proprietary software Geomage. The most important processing steps were the signal processing and the velocity analysis. Without sufficient noise suppression, the velocities would not have been coherent. The results of this work include a set of stacked lines, velocity models and an optimal processing workflow for future high-frequency shallow seismic exploration.
This investigation has proved that surface sourced, shallow seismic surveys can accurately construct a high-resolution cross-section that can distinguish the high and low-velocity sand and shale channels of the Fluvial Wilcox play trend of Northern Louisiana.
Interactive Velocity Analysis and
Stack Imaging
Fold and Shot – Receiver Geometry
Raw shot gather with Frequency spectrum attached. The Top of the Wilcox is highlighted in yellow at ~550 ms.
Data Loading and Preprocessing
Future Work
Because the full processing sequence is still in progress, further steps include depth imaging and pre-stack migration of diffractions. Before this takes place, construction of a more accurate velocity model is necessary. Completion of the full processing sequence allows for interpretation to be carried out and even some specialized procedures, such as AVO, can be applied to examine amplitude anomalies.
Velocity profile of the survey.
Views include Output gather with NMO and stretch mute applied, Location map, Semblance scan, RMS Velocity Field, and Constant velocity stack panels at a 50 m/s increment.
Post-stack signal processing
Acknowledgements
We would like to thank Mark King from King Drilling, Dave Newman, and Doug Temple from United Service Alliance, Anoop William, Jiannan Wang, Christopher Lovely, Andrea Paris, Eliene Silva and Lingfei Mao from University of Houston and Nathan Quick from University of Louisiana at Lafayette.
We would also like to thank Geomage for the use of their processing platform to make all of this possible.
References
Schneider, G. W., 1929, Urania Oil Field, LaSalle, Winn, and Grant Parishes, Louisiana, 1929, SP 3: Structure of Typical American Oil Fields, Volume I
Yilmaz, O., 1987 Seismic Data Processing: Society of Exploration Geophysicists, Tulsa, OK, 525 p.
Sheriff, R. E. 1984, Encyclopedic Dictionary of Exploration Geophysics: 2nd ed.: Tulsa, OK, Society of Exploration Geophysicists, 323 p.