Abstract: Enhanced 3-D Imaging in Difficult Areas from Texas to Yemen, Case History

NICKOLOFF, THOMAS R., Providence Technologies, Inc.; JAMES C. MANATT, JR., Providence Technologies, Inc.

Introduction

Difficult areas for exploration present opportunity to see what has not been seen before, make new plays, and find new reserves by solving signal-to-noise, imaging, sampling, and logistical problems. Problem-solving in difficult areas tends to focus upon deriving incremental benefits achieved through compartmentalization of the process and sometimes through rethinking of the fundamentals.

Breakthroughs, in the seismic sense, are seldom the result of a single step or parameter and are more often the result of a well-calibrated and complete process, from planning through final processing. While we continue to seek the dramatic breakthroughs, seismic imaging advancements in difficult areas more typically come little-by-little.

Extreme topographic and abnormal surface-near surface noise behavior in the Jebel (Plateau)/Wadi (Valley) environments of the Al Masila Block, Republic of Yemen and the Val Verde basin, Texas, respectively, provide fundamental geophysical challenges to imaging and seismic resolution of prolific economic objectives and operating logistics.

The two areas, while nearly half a planet apart, share striking similarities from a geophysical standpoint and can demonstrate how lessons learned in one area can be extrapolated to others.

The geophysical problems related to these difficult environments, often encountered elsewhere in the world, include both surface and subsurface characterizations.

Surface/Near-Surface Challenges

Surface and near-surface challenges to seismic exploration are found in abundance onshore in the form of rugged terrain, coherent noise, and statics problems, both long and short. In general, enhancing the bandwidth of onshore seismic data is an ever-present challenge to geophysicists.

Rugged terrain is well illustrated by the mesa/valley terrain of the Cretaceous Edwards Plateau overlying the prolific Val Verde basin of West Texas and the Tertiary Umm Er Radhuma plateau/wadi terrain of the even more prolific Masila Block, Republic of Yemen, which provide the project and data examples for this paper.

Rugged terrain presents nontrivial operating issues that relate to the logistics of moving hundreds or thousands of receiver groups, MRX boxes, miles of cable and geophones, drills, and dynamite, not to mention the people and communications needed to make precise, accurate placements of this material, then to actually record seismic signal in remote terrain.

Less obvious are the learning experiences that can incrementally improve the overall field effort, such as minimizing source and receiver skips for 3D, and maintaining receiver groups on common elevation and rock outcrop.

In addition to these logistical issues, the hard, high velocity limestones such as those on the surface of much of the Val Verde basin and Masila Block, Yemen, generate significant coherent noise created during seismic acquisition which is commonly known as groundroll, direct wave and head waves. These hard surfaces are referred to as being “high Q” (“Q” being the propensity to ring, the inverse of absorption).

When such a surface is seismically disturbed with a vibrator or a dynamite shot in order to acquire desired, reflected signal from possibly economic subsurface targets, a paradox arises. The same energy initiates the undesired source-generated noise problem and can create significant, high amplitude coherent noise, significant in that it is much greater than the desired signal and can effectively mask subsurface images.

The importance of this, especially in 3D, where we have finite sampling capabilities even with today's high channel acquisition systems and do not have the power of stack array as in 2D, is that if we do nothing physically to address the disparity in the strength of the noise vs. the signal with acquisition parameters, most often with areal geophone arrays, the coherent noise is bound to dominate signal.

When considering this relationship between signal and coherent noise, it is paramount to consider not only the coherent noise traveling along the seismic traverse, but the stone coherent noise which is traveling laterally to a nearby mesa edge or plateau boundary, reflecting back across the receiver detectors. On high velocity, high Q surfaces such as the mesas of the Val Verde and the plateaus of Yemen, this “backscattered” noise, as it is often called, reaches the receiver detectors before much of the desired, reflected signal energy returns from the subsurface target horizon, effectively masking it.

Important aspects to solving the noise problem involves designing arrays of seismic detectors, which act as an antenna, tuned to record the signal and attenuate noise. The utilization of broad dynamic range and high channel recording systems which allow a greater amount of signal to be sampled and distinguished from coherent noise, as well as more-powerful processing algorithms, creative array analysis, and engineered acquisition geometries have improved imaging in difficult areas such as mature provinces like North America and the highly prolific Masila Block of Yemen.

Val Verde Developments

In the Val Verde basin, three giant gas structures, Puckett, Grey Ranch, and Brown Bassett fields, were discovered by major oil companies in the 1950s and 1960s. The discoveries were based primarily on subsurface mapping of structural closures along major regional fault lineaments, weakly supported by occasional glimpses from poor quality 2D seismic reflection or refraction data.

A 1993 discovery by Tom Brown Inc. of Midland and Conoco Inc. of a previously unresolved, unknown Strawn-age thrusted carbonate, just forward of the Ouachita front, ended a long search for another significant new success.

Early multireceiver line tests in 1989 deliberately oversampled the problem in order to allow for robust noise analysis and evaluation of threshold measurements of fold and sampling requirements needed to properly image the subsurface targets. The ability to see and measure this seismic behavior enabled geophysicists to design, engineer, test and prove field receiver and source geometries that effectively attack and help attenuate such problems in the field.

The first commercial applications came in the form of 2D swath techniques. Additional parameter evaluation and modification led to ongoing 3D programs. By carefully extrapolating the lessons from the 2D swath and initial tests, geophysicists were able to design multidirectional receiver arrays, unique field geometries, and resultant, more powerful processing algorithms with which to produce high quality 3D images.

Val Verde Results

These seismic imaging improvements yielded new geologic models, now well documented in the literature, for the basin and led Conoco geoscientists to interpret thrusted beds considerably farther north than previously thought along the Ouachita thrustbelt. Conoco partner Tom Brown announced in early 1993 the ACU 1-49 thrusted Strawn discovery in Terrell County, Texas, with initial flow rates over 14 MMcfd and 300 b/d of condensate at a depth of some 10,000 ft.

The Strawn play continues in exploration and development, supported by new discoveries in allocthonus Permo-Pennsylvanian gas sands. Today, along with Conoco and Tom Brown, Chevron, Mobil, ARCO, Union Pacific Resources, J. Cleo Thompson, Rio-Tex, Enron, Hunt Oil, Hunt Energy, and others successfully drill and shoot 3D seismic over this once poor seismic province. An estimated 150 wells have been drilled in the trend since the Tom Brown/Conoco discovery, with a drilling success rate of 50%, a dramatic success given the complexity.

The Masila Block

Masila Block 14-A in Yemen provides similar opportunities for the industry to record a seismic imaging breakthrough on the basis of many incremental gains.

Canadian Occidental (CanOxy) declared the block commercial in 1991 and started production from it in 1993. In the second quarter of 1997, the Masila fields yielded a mean of 189,400 b/d of oil, 98,500 b/d net to the company.

Unlike in the Val Verde basin, traditional imaging targets in the Masila Block are relatively subtle, not thrusted but complexly overprinted by faulting, and have a stratigraphic component. Successful resolution of these imaging objectives requires broad bandwidth, high quality reflected signal.

The morphology of the block is similar to that of the Val Verde mesas and valleys, with several exceptions. First, the magnitude of the topography is several times greater, with the Tertiary Umm Er Radhuma plateau cliffs often exceeding 250 m. In addition, there exist remnant jebels of Jeza shale on top of these plateaus, with very steep skree-like slopes which must be cut with bulldozers to allow vehicular access.

These challenges require thorough planning, judicious use of bulldozers, and use of combined heliportable and ground vehicle operations, including surveying, drilling, layout, and recording. The key to success was adaptation of lessons from the Val Verde basin, both geophysical and operational, to the extreme conditions of the Masila Block.

CanOxy conducted its first 3D seismic programs on the block in 1995. Although initial results were discouraging, geophysicists were able to analyze the 1995 3D volumes and, with input and insight from the Val Verde, to reassess project goals, parameters, and implementation.

CanOxy discovered a new play in the Jurassic Shuqra carbonate at its Sunah field which supplemented the already significant Cretaceous Qishn clastic play. Unlike the Qishn reservoirs, which in general are gentle faulted structures, the deeper Shuqra reservoirs underlie very high impedance Sarr and Naifa carbonates in half graben geometry, requiring the imaging of steeper dips, deep fault patterns, the basement, and stratigraphic thinning. An additional objective was to tie deeper faults with known, mapped surface faults.

While the 1995 Sunah 3D seismic survey data recovered spotty images of the Qishn, no Shuqra images raised questions as to whether the formation could be imaged, given the significant source-generated noise in the area and topography.

A distinct anomaly was a key 2D line previously recorded, oriented in dip direction along a large, north-south wadi, which nicely imaged the half graben geometry. In 1996, with a Val Verde approach, the 2D and 1995 3D data were reprocessed and analyzed with previous source tests in order to recharacterize the coherent noise effects on signal.

Striking similarities to the Val Verde noise components were observed. Speeds of the head waves, direct waves, and groundroll were almost identical, half a planet away, and backscattered energy was apparent in the shot records. Geophysicists recognized that Val Verde-type array and field geometry strategies could be brought to bear on the problem.

Therefore, the goal of the design team became basically threefold. First, the significant coherent source-generated noise and its backscatter had to be addressed, and previous array work had to be tuned to Masila conditions. Next, target depths, sampling requirements, and known source and receiver skips were added to the equation in order to optimize certain parameters, such as azimuth and offset distribution, that would enhance powerful modern 3D processing techniques such as 3D dip moveout (DMO), beam steering, and migration. Finally, the design was to be adaptable and capable of implementation on the rough Masila Block.

Differences that needed to be incorporated in the parameters were the shallower depth of the objectives, the overall faster subsurface average velocity, and the topographic and logistical issues. Source and receiver arrays were set through field startup testing. Charge sizes had to be carefully analyzed because of the variability of surfaces and the natural limitations on use of areal arrays. Crew members compiled an array bible ranking options for various topographic and surface conditions. Several crews painted rocks for each source and receiver array, keeping a detailed log of them all.

Analysis of the placement of contributory source and receiver locations that needed to be offset required several man-weeks of effort. Determining how to achieve best subsurface coverage required several iterations of source and receiver placement along with analysis of fold, offset, and azimuth coverage. Surveys used a combination of conventional and RTK, GPS equipment, often assisted by helicopters in remote locations.

The 3D high channel acquisition system used in the survey maintained and recorded 1,250-plus live channels at a time. The need for reliable equipment was illustrated by one particularly onerous receiver line that traversed, with cables, six deeply cut orthogonal wadis, resulting in 12-13 up or down traverses ranging from 150 to 250 m each.

Masila Block Results

Final results from the 1996-97 Sunah 3D survey helped define the geometry of the half graben structures, allowed for pursuit of additional Shuqra targets, and produced a sharper, broader bandwidth image of the Qishn reservoirs above. Shot concurrently with the 1996 3D program was a 2D swath seismic program primarily for regional definition and additional field delineation. The 1996 2D and 3D project on Masila Block 14-A included approximately 150 sq km of 3D, 150 km of 2D, and 9 km of test lines.

Additional drilling opportunities have prompted CanOxy to add a second full-time drilling rig. During first quarter 1997, in part due to 1996 2D swath seismic, CanOxy drilled three delineation wells and successfully expanded the Tawila field. Together, the wells flowed 24,000 b/d of oil, boosting gross field output to 70,000 b/d.

Additional activity has been reported at the Camaal field, site of further 2D swath seismic work in 1996. Drilling at Camaal has yielded 3 additional wells with expected combined production rates of 18,000-28,000 b/d. The Heijah field, also a site of new 1996 3D data activity, and the Sunah field have also reported additional activity.

It was reported in September that an additional 76 million bbl proved reserves net to CanOxy has been provided by production performance, development, and seismic information, boosting total net reserves to 169 million bbl. In addition, reserves life has been extended by 5 years.

CanOxy has announced plans to acquire up to 250 sq km more seismic data coverage in 1997 to further delineate its Masila holdings.

Conclusions

Improvements to seismic imaging in difficult areas enable companies to extend the limits of resolution with potentially big results.

The methods used in the Val Verde basin, for example, can apply to un- or underexplored thrustbelts throughout North America. The onshore overthrust margin incorporates several active wildcat plays including the extensive Canadian Foothills trend along with the producing thrustbelt trends of the Val Verde and Arkoma basins of the Lower 48. More often than not, these plays are poorly imaged due to surface and subsurface problems.

U.S. Geological Survey data suggest there may be 39 tcf of undiscovered gas and 2.2 billion bbl of oil in remote overthrust regions of Alaska—the largest postulated but undiscovered volumes within a discrete onshore trend on the North American continent. Some 5 tcf of gas potential is ascribed to the thrustbelt of the western Green River basin, Wyoming. And thrust trends of unknown future potential exist in the Rockies, Central Texas, Alabama, and the Appalachians all the way to Newfoundland.

Results from Yemen's Masila Block show how rethinking the 3D acquisition design and geometry for formidable area-dependent problems can enhance imaging and improve understanding of both proven reservoirs and new exploration opportunities with world class potential. The Masila fields are the most prolific in the country, exceeding the Exxon/Hunt block. Combined production from the Masila fields rose from 171,000 b/d in 1996 to 185,000 b/d in the first quarter of 1997.