Subsalt Exploration Research Articles

Basin-range coupling relationship and prediction of favorable targets for ultra-deep subsalt exploration in the Tarim Basin, Northwest China

The Tarim Basin holds significant geological interest due to its complex basin-range coupling relationship and promising prospects for hydrocarbon exploration. The basal Cambrian in this basin records all tectonic movements of surrounding plates and influences the distribution of source rocks, reservoirs and caprocks. Its morphological evolution not only reflects changes of the uplift-depression framework but also indicates the migration pathways of hydrocarbons. Based on seismic profile interpretations, the structural morphologies of the basal Cambrian and active faults in various geological periods are obtained and related to regional plate tectonics. Furthermore, favorable targets for ultra-deep hydrocarbon exploration are predicted by identifying structural hinge zones. The results show that, (1) the initial amalgamation of the Tarim Plate with South Kunlun Terrane in the southwest at the end of the Middle Ordovician led to uplift of the Southwestern Depression, Bachu uplift, and Tazhong uplift, with the formation of NW-SE-trending boundary faults, (2) the amalgamations of the Tarim Plate with South Kunlun Terrane in the southwest, and Altun-Qilian Terrane in the southeast, as well as the bidirectional subduction of the South Tianshan Ocean in the north at the end of the Ordovician led to uplift of the Southwestern Depression, Bachu uplift, Tazhong uplift, southeastern basin, and Tabei Uplift, with the development of the Bachu-Tazhong fault system trended NW-SE, Madong back-thrust fault system trended NE-SW, and Tabei fault system trended E-W, (3) the amalgamation of the Qaidam Block and Tarim Plate in the southeast and the continued subduction of the South Tianshan Ocean in the north at the end of the Silurian led to uplift in the Madong structural belt, Gucheng low uplift, Tadong low uplift, Shuntuoguole low uplift, and Tabei Uplift, with a large-scale reactivation of the Madong back-thrust fault system and small-scale activities of the Bachu-Tazhong and Tabei fault systems, (4) the closure of both the North and South Tianshan Oceans in the Carboniferous and the intense uplift of the Tianshan orogenic belt in the Permian in the north, as well as the amalgamation of the Tarim Plate with Tianshuihai Terrane in the southwest, led to the development of the Tabei Uplift, Shuntuoguole low uplift, Awati sag, northwestern Bachu uplift, Tazhong uplift, and Southwestern Depression, with the strong activities of the Tabei and Bachu-Tazhong fault systems, and weaker activity of the Madong back-thrust fault system, (5) the amalgamations of the Tarim Plate with North Qiangtang Block, South Qiangtang Block, and Lhasa Block during the Mesozoic in the southwest led to uplift of the Southwestern Depression, Bachu uplift, Tazhong uplift, and Tadong low uplift, with the strong activity of the Bachu-Tazhong fault system, (6) the remote effect resulting from the collision between the Indian and Eurasian plates in the Cenozoic led to significant uplift of the Bachu uplift and varying degrees of subsidence in other basin areas, with intense fault activities in the southwestern, northwestern, and northern regions. Based on the distribution of Middle-Lower Cambrian source rock, reservoir, and caprock, the favorable targets for ultra-deep subsalt exploration include the central-eastern Tazhong uplift, central Lunnan low uplift, central Shuntuoguole low uplift, southwestern Markit slope, northwestern Yecheng sag, and northwestern Bachu uplift.

Enhancing subsalt imaging through advanced identification and suppression of interbed multiples and mode-converted reflections — Gulf of Mexico and Brazil case studies

In our case studies, the success of subsalt exploration and development wells depended heavily on the characterization of highly heterogeneous lacustrine microbial carbonates. Acoustic and elastic inversions have proved to be a good proxy for identification of reservoir quality variation for exploration and development well placements. However, qualitative and quantitative usage of subsalt seismic amplitudes requires proper illumination and good signal-to-noise ratio. If properly imaged, mode-converted reflections and interbed multiples can be complementary to the P-wave image. But, in conventional P-wave-oriented imaging, both types of events cannot be imaged correctly. They appear as coherent noise and negatively impact the overall exploration and development project outcomes, especially in areas with poor illumination. This paper consists of two parts: first, we investigate the potential problems resulting from converted waves and interbed multiples in data from two different basins — the Gulf of Mexico and the Campos Basin in offshore Brazil — and show our approach to attenuate them to reveal the true structures. The second part focuses on advanced identification of interbed multiples in modeling and migration methods. To facilitate the various strategies to attenuate interbed multiples, “interpretation” of the various events plays a significant role. Vertical seismic profile (VSP) data are excellent for the purpose; however, these data are only available at well locations, if they are recorded. As a result of many years of technology advancement, pseudo VSP data can be constructed effectively from standard streamer survey data. Two methods are highlighted in this paper for building pseudo VSP data in a full two-way sense, based on a typical Brazil-type salt model: Marchenko-based processing and full-wavefield migration. Major subsalt plays in the Gulf of Mexico and emerging plays in Brazil should benefit significantly from elimination of these kinds of coherent noise.

Study on tectonic and sedimentary evolution during the Nanhua-Middle Cambrian and its significance for subsalt exploration, Tarim Basin#br#

Study on tectonic and sedimentary evolution during the Nanhua-Middle Cambrian and its significance for subsalt exploration, Tarim Basin#br#

Deep Mediterranean's Messinian evaporite giant: How much salt?

Abstract Evaporite giants can precipitate out of high salinity waters in semi-enclosed ocean basins when the rate of evaporation outstrips the inflow of new saline and fresh waters. The thick saline series deposited in the deep Mediterranean during the Miocene Epoch's late Messinian age (between 5.97 and 5.3 Ma) is among the youngest of such salt giants in Earth history. During this event, also known as the Messinian Salinity Crisis, voluminous evaporites accumulated in the deep basins when the main inflow of saline water to the Mediterranean through the Gibraltar Straits was severely diminished. The evaporites are interbedded with sediments eroded from the shelves and slopes or mass-transported downslope. Recent advances in geochronology and better understanding of the geodynamic history of various basins have led to insights about the timing of the salinity crisis and the tectonics and role of Gibraltar passage and other western narrows from the Atlantic to the Mediterranean. New estimates based on ocean-wide seismic facies analysis presented here reveal that there is between 821 ± 50 and 927 ± 50 thousand cubic km of late Messinian salt, and a total of up to 1.2 ± 0.1 million cubic km of salt plus associated sediment tied up in the deep Mediterranean basins. This suggests that after the initial restriction the Mediterranean had to be either continuously supplied with brine, or partially to completely refilled several times to produce the total salt edifice. The amount of Atlantic saline water needed to amass this evaporite giant is between 7 and 8 times the modern-day Mediterranean's equivalent of saline water. Our new volumetric estimates have implications for the Mediterranean's Messinian sequestration and desiccation scenarios and should lead to more meaningful geodynamic models and provide constraints for many of the controversies that still surround this major event in geological history. Maps of salt distribution in various basins also have important implications for sub-salt exploration geoscience.

Subsalt exploration in shallow waters of the Republic of Congo

Subsalt exploration in shallow waters of the Republic of Congo

Lower permian reef-bank bodies’ characterization in the pre-caspian basin

Reef-bank reservoir is one of the targets for exploration of marine carbonate rocks in the Pre-Caspian Basin. Within this basin, the reef-bank bodies were primarily developed in the subsalt Devonian-Lower Permian formations, and are dominated by carbonate platform interior and margin reef-banks. The Lower Permian reef-bank present in the eastern part of the basin is considered prospective. This article provides a sequence and sedimentary facies study utilizing drilling and other data, as well as an analysis and identification of the Lower Permian reef-bank features along the eastern margin of the Pre-Caspian Basin using sub-volume coherence and seismic inversion techniques. The results indicate that the sub-volume coherence technique gives a better reflection of lateral distribution of reefs, and the seismic inversion impedance enables the identification of reef bodies’ development phases in the vertical direction, since AI (impedance) is petrophysically considered a tool for distinguishing the reef limestone and the clastic rocks within the formation (limestone exhibits a relatively high impedance than clastic rock). With this method, the existence of multiple phases of the Lower Permian reef-bank bodies along the eastern margin of the Pre-Caspian Basin has been confirmed. These reef-bank bodies are considered good subsalt exploration targets due to their lateral connectivity from south to north, large distribution range and large scale.

3D Gravity Inversion using Tikhonov Regularization

Subsalt exploration for oil and gas is attractive in regions where 3D seismic depth-migration to recover the geometry of a salt base is difficult. Additional information to reduce the ambiguity in seismic images would be beneficial. Gravity data often serve these purposes in the petroleum industry. In this paper, the authors present an algorithm for a gravity inversion based on Tikhonov regularization and an automatically regularized solution process. They examined the 3D Euler deconvolution to extract the best anomaly source depth as a priori information to invert the gravity data and provided a synthetic example. Finally, they applied the gravity inversion to recently obtained gravity data from the Bandar Charak (Hormozgan, Iran) to identify its subsurface density structure. Their model showed the 3D shape of salt dome in this region.

Accumulation conditions and exploration directions of natural gas in deep subsalt Sinian-Cambrian System in the eastern Sichuan Basin, SW China

Abstract Based on comprehensive analysis of outcrops and drilling data, and interpretation of more than 6×10 4 km 2 seismic data, this paper delineates the regional distribution of the gypsum-salt layers in Middle-Lower Cambrian in eastern Sichuan Basin, and discusses accumulation conditions for natural gas in deep subsalt Sinian-Cambrian System in eastern Sichuan Basin, and points out favorable exploration directions. There are two sets of regional gypsum-salt layers in the Middle-Lower Cambrian in eastern Sichuan Basin, which are regional detachment layers, with fault-related folds occurring above and broad anticlines occurring below. There are 6-7 rows of NNE and NE trending large traps developed under the salt. Good space configuration of source rocks, reservoirs and cap rocks provide favorable accumulation conditions for natural gas in the deep subsalt in this region: three sets of hydrocarbon source rocks including Sinian Doushantuo Formation, the third Member of Sinian Dengying Formation, and dominant Cambrian Shuijingtuo Formation; three sets of weathering crust karst reservoirs and grain beach reservoirs including the second and fourth Members of Sinian Dengying Formation, and Cambrian Shilongdong Formation; and the regional cap of gypsum-salt in the Middle-Lower Cambrian and shale in Shuijingtuo Formation. The top target in deep subsalt exploration in eastern Sichuan Basin should be the fourth Member of Dengying Formation, followed by Shilongdong Formation and the second Member of Dengying Formation, and areas on migration direction with appropriate burial depth and being far away from the edge of the basin should be the first choices in exploration. Therefore, the favorable hydrocarbon exploration areas in the near future are the Honghuadian-North Huaying Mountain, Liangshuijing-Pubao Mountain-Leiyinpu, and Datianchi structural belts.

Kinematic linkage between minibasin welds and extreme overpressure in the deepwater Gulf of Mexico

A geopressure interpretation technique known as the seismic velocity method is a common workflow in which shale compaction functions are characterized at offset control wells, matched to interval seismic velocities, and then used to predictively calculate geopressure away from well control. The seismic velocity method is used to interpret the expected geopressure profile at the Deep Blue subsalt exploration well in Green Canyon 723 in the deep water Gulf of Mexico. The Deep Blue prospect is distinct from other prospects in the play fairway in that the prospective section is overlain by a salt withdrawal minibasin, whereas the offsetting fields are positioned either along the flanks of minibasins or under a thick allochthonous salt canopy. Predrill geopressure interpretations using numerous tomographic imaging velocity data sets shows a large degree of consistency with the magnitude of geopressure encountered in offsetting supra salt and subsalt fields. Results from the Deep Blue 1 exploration well indicate the predrill geopressure interpretation from interval seismic velocities failed to anticipate the extreme degree overpressure encountered in the subsalt section of the well due to poor deep velocity resolution and an “unloaded” compaction signature. The magnitude of overpressure in the primary section is attributed to the emplacement of an unconformable halokinetic sequence over the primary subsalt basin. An interpretive paradigm is described in which the Deep Blue pressure cell is created through two halokinetic episodes: (1) rapid progradation of a salt canopy followed by (2) subsequent salt withdrawal and emplacement of an overlying minibasin. The linkage between halokinetic sequences, burial history, and the development of overpressure can be used to predictively characterize subsalt geopressure environments.

Post-Macondo Drilling in Deepwater Gulf of Mexico Faces Added Challenges

This article, written by Editorial Manager Adam Wilson, contains highlights of paper SPE 154928, ’Challenges Associated With Drilling a Deepwater, Subsalt Exploration Well in the Gulf of Mexico: Hadrian Prospect,’ by Mark C. Moyer, SPE, Scott B. Lewis, SPE, Mike T. Cotton, SPE, and L. Miles Peroyea, SPE, ExxonMobil, prepared for the 2012 SPE Deepwater Drilling and Completions Conference, Galveston, Texas, 20-21 June. The paper has not been peer reviewed. The Hadrian-5 prospect in the US Gulf of Mexico was drilled in approximately 7,000 ft of water as one of the first Gulf of Mexico wells drilled after a deepwater moratorium was imposed after the Macondo disaster. This exploration well was permitted under the new regulatory requirements of the US Bureau of Ocean Energy Management, Regulation, and Enforcement (BOEMRE). In addition to well-design regulations, BOEMRE required operators to calculate a worst-case discharge scenario and to develop containment plans for that scenario. The well was drilled and evaluated in a safe manner with no significant incidents, even though the geologic formations encountered were different than predicted. Introduction As a result of the April 2010 Macondo disaster, the US government responded with a moratorium on deepwater drilling, reorganized its regulatory agency, and is-sued new rules for offshore drilling. In November 2010, BOEMRE is-sued the Notice to Lessees and Operators 2010-N10. This notice to lessees and operators (NTL) required operators to certify compliance with all applicable regulations and submit plans to contain a worst-case discharge scenario as part of the drilling permit application. The Hadrian well is located in Block 919 of the Keathley Canyon in 6,941 ft of water, approximately 220 nautical miles southwest of the shorebase location in Fourchon, Louisiana (Fig. 1). The purpose of this exploration well was to evaluate the potential resource of the subsalt Pleistocene, Pliocene, and Miocene formations. On the basis of seismic data, there were no shallow hazards and no hydrocarbons above the salt structure. The Hadrian-5 well had an approved permit and a rig on location at the time of the moratorium, but the work was suspended and the rig was released to work on the Macondo spill response. The drill team, in conjunction with the geoscience and regulatory organizations, revised the exploration plan and the application for permit to drill (APD) beginning in July 2010 to meet the new regulatory requirements. BOEMRE approved the revised permit to drill in March 2011. Planning and Permits Following the Macondo disaster, BOEMRE issued additional requirements that operators must fulfill before regulatory approval is granted. Additional work was required during the planning and permitting phase to meet these new requirements. Interim final rules issued by BOEMRE required operators to submit additional details as part of the APD. Blowout preventer (BOP) equipment was one area of focus for the new rules. BOP control system drawings and independent certification of the BOP equipment design and capability must now be submitted. The Hadrian team worked closely with the rig contractor, and an independent agency provided the required documentation.

Orthogonal wide azimuth surveys: acquisition and imaging

Wide azimuth seismic surveying is firmly established as a key element in subsalt exploration and production in the Gulf of Mexico. The challenge now facing the industry is to further improve subsalt images in areas with existing wide azimuth coverage. Orthogonal wide azimuth acquisition, acquiring a second wide azimuth dataset over existing wide azimuth data, is a technique that provides close to full azimuth coverage while utilizing existing wide azimuth data. A case study performed in the Gulf of Mexico demonstrates that the increased source and receiver illumination created by orthogonal wide azimuth surveys significantly improves imaging results and increases the effectiveness of the tomography and salt interpretation steps.

Subsalt exploration and development — Introduction

This special section on subsalt imaging originated from a summer research workshop on the same topic held 26–29 July 2010 in Lake Tahoe, Calif. The title of the workshop, “Subsalt exploration and development: Four years later … what’s new in acquisition, imaging and interpretation,” describes exactly the variety of topics presented. In some sense, this section attempts to cover most of these topics. Though not all topics or presentations given at the workshop made it into this special section, a great majority of the ideas discussed are represented here. Several papers were also submitted to this special section that were not presented at the workshop. The section opens with a general review of subsalt imaging meant to be accessible by a wide audience. Leveille et al. address the question of what subsalt imaging is and discuss the physics of the subsalt imaging problem, focusing on the illumination issue. It also provides a review of the main imaging algorithms, the various imaging conditions, and the velocity updating tools. Six general case history papers follow the review paper. In the first paper, to learn about the accuracy of anisotropic deep-water Gulf of Mexico model building and depth-imaging tools that are used for the processing and imaging of acquired field data, Hoxha et al. create a 3D VTI anisotropic earth model and a 3D seismic data set representing typical subsalt Gulf of Mexico geology called Tempest. The Tempest reverse time migration PSDM is being used to learn about the differences between the exact geological model and the reverse time migration PSDM image, helping in the interpretation of real reverse time migration prestack depth migrated data. Following this, Zhou et al. analyze …

Subsalt imaging for exploration, production, and development: A review

The field of subsalt imaging has evolved rapidly in the last decade, thanks in part to the availability of low cost massive computing infrastructure, and also to the development of new seismic acquisition techniques that try to mitigate the problems caused by the presence of salt. This paper serves as an introduction to the special Geophysics section on Subsalt Imaging for E&P. The purpose of the special section is to bring together practitioners of subsalt imaging in the wider sense, i.e., not only algorithm developers, but also the interpretation community that utilizes the latest technology to carry out subsalt exploration and development. The purpose of the paper is in many ways pedagogical and historical. We address the question of what subsalt imaging is and discuss the physics of the subsalt imaging problem, especially the illumination issue. After a discussion of the problem, we then give a review of the main algorithms that have been developed and implemented within the last decade, namely Kirchhoff and Beam imaging, one-way wavefield extrapolation methods and the full two-way reverse time migration. This review is not meant to be exhaustive, and is qualitative to make it accessible to a wide audience. For each method and algorithm we highlight the benefits and the weaknesses. We then address the imaging conditions that are a fundamental part of each imaging algorithm. While we dive into more technical detail, the section should still be accessible to a wide audience. Gathers of various sorts are introduced and their usage explained. Model building and velocity update strategies and tools are presented next. Finally, the last section shows a few results from specific algorithms. The latest techniques such as waveform inversion or the “dirty salt” techniques will not be covered, as they will be elaborated upon by other authors in the special section. With the massive effort that the industry has devoted to this field, much remains to be done to give interpreters the accurate detailed images of the subsurface that are needed. In that sense the salt is still winning, although the next decade will most likely change this situation.

Salt gets in your eyes: the geological challenges and solutions to sub-salt exploration

Salt gets in your eyes: the geological challenges and solutions to sub-salt exploration

Well-to-seismic tie method in complex imaging areas: Examples in the deep offshore subsalt Angola domain

Recently, imaging capabilities have allowed a major step forward in subsalt exploration. In particular, new PSDM algorithms have appeared on the market that account for complex wave propagation through salt canopies to produce better images. These improvements have allowed the drilling of numerous subsalt exploration wells, but at the time of appraisal and development, questions arise on the reliability of these seismic cubes to accurately predict geometries. This issue is crucial in the offshore deepwater blocks of Angola, where reflectors often have high dips and subsalt seismic visibility is not optimal. Furthermore, velocity models used for subsalt imaging are seldom as accurate as one could want, and significant discrepancies can often be observed between seismic and well depth of geological markers. These discrepancies can have several origins, such as neglecting to account for anisotropy, as discussed by Shumaker et al. (2007). The lack of accuracy of velocity models can also be due to the limitations of velocity estimation techniques in the subsalt domain given the available data. This results in a difference between the depth of markers observed in the well (true depth) and on seismic images (seismic depth). It is to be noted that the seismic depth is dependent on the PSDM velocity model to the first order, but also on the migration algorithm because of different ways to model the wave propagation (ray tracing or wavefield extrapolation).

Migration velocity analysis for tilted transversely isotropic media

ABSTRACTTilted transversely isotropic formations cause serious imaging distortions in active tectonic areas (e.g., fold‐and‐thrust belts) and in subsalt exploration. Here, we introduce a methodology for P‐wave prestack depth imaging in tilted transversely isotropic media that properly accounts for the tilt of the symmetry axis as well as for spatial velocity variations.For purposes of migration velocity analysis, the model is divided into blocks with constant values of the anisotropy parameters ε and δ and linearly varying symmetry‐direction velocity VP0 controlled by the vertical (kz) and lateral (kx) gradients. Since determination of tilt from P‐wave data is generally unstable, the symmetry axis is kept orthogonal to the reflectors in all trial velocity models. It is also assumed that the velocity VP0 is either known at the top of each block or remains continuous in the vertical direction. The velocity analysis algorithm estimates the velocity gradients kz and kx and the anisotropy parameters ε and δ in the layer‐stripping mode using a generalized version of the method introduced by Sarkar and Tsvankin for factorized transverse isotropy with a vertical symmetry axis.Synthetic tests for several models typical in exploration (a syncline, uptilted shale layers near a salt dome and a bending shale layer) confirm that if the symmetry‐axis direction is fixed and VP0 is known, the parameters kz, kx, ε and δ can be resolved from reflection data. It should be emphasized that estimation of ε in tilted transversely isotropic media requires using nonhyperbolic moveout for long offsets reaching at least twice the reflector depth. We also demonstrate that application of processing algorithms designed for a vertical symmetry axis to data from tilted transversely isotropic media may lead to significant misfocusing of reflectors and errors in parameter estimation, even when the tilt is moderate (30°). The ability of our velocity analysis algorithm to separate the anisotropy parameters from the velocity gradients can be also used in lithology discrimination and geologic interpretation of seismic data in complex areas.

Introduction to this special section: Subsalt exploration

Salt basins occur worldwide (Figure 1), and large hydrocarbon deposits are often associated with the presence of salt structures due to favorable conditions for accumulation and trapping of oil and gas. Because of the cost and difficulty of drilling subsalt, it is important to be able to image subsalt prospects correctly. However, the large velocity contrast between the salt and sediments, and the complexity of many salt bodies, make subsalt imaging a difficult task. A step-change in technology is therefore required, and two recent workshops have been held to address the issues. The “Workshop on Subsalt Imaging Problems: Practical Problems Seeking Solutions” was held at the Colorado School of Mines in 2005, during which presenters from oil and service companies focused on the difficulties of subsalt imaging (Leveille et al., TLE 2005). The 2006 SEG/EAGE Summer Research Workshop on “Subsalt Exploration and Development: Imaging, Interpretation, and Drilling—What have we learned?” covered a wide range of sal...

Subsalt exploration potential of the Moroccan salt basin

The Moroccan salt basin is one of the remaining underexplored West African salt basins. The dimensions of this salt basin are about 100 km in average width and almost 1000 km in length along the Atlantic margin of Morocco (Figure 1). As opposed to the Aptian salt basins of the South Atlantic, in northwestern Africa the salt was deposited during the last stage of rifting, prior to continental breakup, in the Upper Triassic/Lower Jurassic (Hafid, 2000; Tari et al., 2003).

The role of alternative scenarios and a regional geologic framework for subsalt and deep exploration

Over 50 years ago Wallace Pratt, a preeminent geologist with the Humble Oil and Refining Company, had it right when he said “Where oil is first found is in the minds of men…” Today, many minds—of both men and women—are finding exploration opportunities by developing new, innovative exploration concepts based on regional understanding combined with breakthrough imaging technologies.

Subsalt Exploration and Development: Imaging, Interpretation, and Drilling—What have we learned? 2006 SEG/EAGE Summer Research Workshop

This year's Summer Research Workshop organized by SEG and EAGE under the title “Subsalt Exploration and Development: Imaging, Interpretation, and Drilling—What have we learned?” took place on 10–14 July 2006 in Snowbird, Utah, USA. The workshop was attended by approximately 150 geophysicists and geologists representing the whole spectrum of oil and gas companies, geophysical contractors, and academia. The workshop featured many presentations highlighting the current state-of-the-art in subsalt exploration. The large majority of the presentations focused on the use of surface seismic data to guide subsalt exploration and development. However, there were significant contributions that discussed the role of geology, geomechanics, and borehole data in constraining model building under salt bodies characterized by complex geometries (Figure 1).