Maximising the Opportunity with FTG Gravity
Colm A. Murphy
Chief Geoscientist
Bell Geospace Limited, Edinburgh, UK

The current low risk, low price environment efficiencies to maintain best practices for oil and gas exploration. Cheap oil and gas means companies are faced with increasing challenges to honour license commitments from exploration, to appraisal, to field development. Government licensing authorities, exploration companies, contractors and consultants are all faced with challenging decisions and work practices to meet expectations. However, if anything has been learned from the recent boom year sit's that new technologies, of fering improved resolution and successful returns, continue to be developed and exploited.

New technologies offer advanced capabilities to maximise opportunities. Their increased resolving power allows for a more far-reaching capability to detect, delineate and map prospects with incredible resolution. These technology advances have been largely in seismic and drilling capabilities and benefit enormously with increased computing power through design, implementation and processing know-how. However, many of these also involve increased expense leading to challenging decisions for exploration managers in the current climate. Do we continue to spend, do we reduce capability, and do we reduce the work program? How does one promote new acreage using these new technologies in a low price environment? How does one streamline prospects with limited funds to optimise their holdings? These are just a handful of questions facing such managers.

Many companies have looked to alternative technologies to help mitigate against this imposition and to maintain their work programs. Airborne geophysics has seen increased uptake in recent times and rapid advances have been made, with increased resolution now the norm. Airborne magnetics, airborne EM and airborne gravity gradiometry have all made a break through with their ability to detect, delineate and map prospectively at an unprecedented scale in recent years.

Airborne gravity gradiometry is arguably the most successful of these new technologies. Gravity gradiometry maps the gravity field as recorded by sub -surface geology and presents an order of magnitude greater resolving power over its conventional counterpart, Gravity.

Airborne Full Tensor Gradiometer (FTG) goes one step further in mapping the 3D Gravity field and has witnessed a rapid uptake by the Oil & Gas exploration industry since its ground-breaking work in the Albertine Basin with Tullow Oil in Uganda back in 2009 where 26 out of 27 successful wells drilled were FTG anomalies.

The Full Tensor and 3D Gravity field

FTG comprises 12 gravity measuring devices arranged in 6 pairs with 2 pairs on each of three separate spinning discs. Each spinning disc ensures the field is measured in all directions of the 3D field and all at the same time on any single survey. The technology detects density contrasts lying both directly beneath and in between sur vey lines, a feat unmatched by any other gravity measuring technology. The combined output measures the rate of change of the gravity field as detected in sub-surface geology at an unprecedented resolution. The stark difference with conventional gravity measuring devices is that unwanted signal, in the form of platform contributions to the signal, is cancelled out. This facilitates measurement of the gravity effect of subtle density contrasts sourced by subtle geological changes in the shallow section.

The different outputs are transformed into a Gravity Gradient Tensor field that allows direct detection of complex geological body shapes as they present themselves in a typical geological setting. Such features include closed fault blocks, anticlinal closures, salt bodies, carbonate mounds from many intra-basinal settings, but also include mapping primary basin forming trends and regional tec tonic settings from strike -slip, normal and reverse faults. This combined high accuracy, high resolution output facilitates mapping of the 3D subsurface geology on any given survey.

FTG data is acquired from both sea-going vessels and fixed wing aircraft. Airborne acquisition is challenging, but survey work conducted on stable, slow moving platforms such as a Basler Turbo aircraft (Figure1) offer immense value. The long wingspan and high endurance capability ensures acquisition of highly accurate, high resolution data in a timely fashion. Bell Geospace operates three such aircraft worldwide, often citing the aircraft's unique offering as a key driver in their ability to deliver high quality FTG data over the years.

Figure 1: Basler Turbo 67 aircraft used for acquisition of FTG data by Bell Geospace

Early successes with FTG

Some of the first FTG projects were offshore in the Gulf of Mexico, North Sea, and Norway's Barents Sea in the 1990's and early 2000's where salt models were defined for seismic workflows.

FTG helped depic t accurate depth and body shapes of complex salt bodies, including overhang development and connectivity with deeper mother salt. Density-depth models proved to be of immense use for early stage PSDM workflows that ser ved to advance seismic imaging exercises.

Sub-basalt imaging with FTG was also achieved offshore in the Faroe Islands at the turn of the century. Largescale basaltic lava flows are characterised by a laterally homogenous density expression making them blind to gravity surveying methodologies. The impact is that high resolution gravity sur veys image sub-basalt geology accurately. High resolution FTG established depth to tops of the key Mesozoic fault blocks underpinning trapped hydrocarbon occurrences and were confirmed by drilling on Statoil's Brugdan prospect in 2006.

FTG for the ages
The key focus for FTG since then has been in delineating intra-basinal structuring in many play model environments: from Carbonates and Thrust-Fold belts in SE Asia and India, to back-arc and fore-arc basins in the Americas, southern Europe, and the Middle East, to Rift Basins in Europe and Australia to fault mapping in unconventionals exploration in the US and Australia. FTG's unique ability to simultaneously measure all components of the gravity field means it is best suited to mapping the 3D shape of geological structures generating density contrasts.

Tullow Oil's success in 2009 with FTG is remarkable. The target geological structures were a series of tilted faulted blocks. Cored by basement rock and overlain by lacustrine sediments and shales, their density contrast is ideal for detection. Conventional gravity data acquired initially established the concept model, but as much of the acreage is over difficult to reach parts, an airborne solution was sought. The 26 out of 27 successful wells drilled by 2009 are underpinned by positive FTG anomalies (Fig2) with the only duster being associated with a negative anomaly.

FTG's success in Uganda lead to extensive survey work elsewhere along the East African Rift from Ethiopia to Kenya, from Malawi to Mozambique and activity is still ongoing with recent surveys just completed in Tanzania and Zambia. The uptake since 2009 has been immense, with Petronas, Repsol, Sasol , and BG, to name a few, adopting the technology in their exploration workflows. Delineation of key rifted segments and their complex structuring are confirmed by sparse seismic coverage where available and used to steer planning of new seismic acquisition, both 2D and 3D. The value is efficient use of exploration budget at a time when costs are everything.

Figure 2: FTG mapping hydrocarbon bearing structures in the Albertine Basin, Uganda. Image reproduced from joint Africa Oil and Tullow Oil presentation at UBS Global Oil & Gas Conference, 2012.

Petronas in Malaysia committed in 2012 to FTG survey work over large areas both onshore and offshore Peninsular Malaysia, Sarawak and Sabah. 4 years and 162,000 sq kms of FTG data later, the data coverage represents the largest single holding of FTG by any operator. The diverse and complex array of anomaly patterns is testament to the technologies' ability to resolve some of the complex geological exploration challenges in these parts.

The Sarawak Basin is well established as an oil producer with many producing fields from the carbonate platform since the 1970's. The current focus is to now image sub-carbonate to identify and delineate new plays. The carbonate properties and body shapes are a challenge for conventional seismic workflows leading to long processing times. FTG does image sub-carbonate, identifying basins and migratory pathways that feed the productive reser voirs within the carbonate itself. Figure 3 shows a depth to top Pre-Cycle Basement generated from FTG data over the Luconia platform offshore Sarawak. The FTG Depth Imaging workflow establishes a variable depth to the dominant metasedimentary basement and predicts the presence of pre-Carbonate basins reaching 4km depths(cold colours). The additional benefit is direct mapping of key structural lineaments interpreted as faults predicting both transfer and normal fault activity. These ser ve as migratory pathways for generated hydrocarbons from depth into the overlying Carbonate hosted reservoirs.

West of Luconia in what is known as the Half-Grabens of the Tatau region, the geology changes from dominant carbonate platform to more conventional rift basin with the development of a series of half-grabens. Plays have been successfully drilled with two oil accumulations from well NNG-1 (Jabber et al, 2015). The deeper of the two pools resides at Top Basement defined by a tilted fault block structure. Corresponding FTG data facilitates direct mapping of the structure (Figure 4) away from that depicted in the seismic. The negative anomaly pattern coincides perfectly with the basinal troughs where the source rocks reside.

Figure 4: FTG response over a successfully drilled half-graben structure offshore Malaysia (seismic from Jabber, 2015)

Thrust and fold-belt settings are ideal hunting grounds for FTG applications . The north-western shores and onshore Sabah, Malaysia, has received much interest with regional 2D seismic and geological map data being the primary technologies used to delineate thrust faults, anticlines, and synclines. FTG data acquired in 2015 confirms the published interpretations of Cullen (2010 ) with positive anomalies tracking the thrust/reverse fault patterns and cold colours the known synclines. Figure 5 shows the one - on- one correlation with corresponding fault picks made from 2D seismic data. The availability of the FTG data now ensures confident mapping of the syncline's extent previously only predicted from the seismic.

Figure 5: FTG response over a previously mapped thrust fault and syncline. Grey lines locate composite seismic section with white fault lines coinciding with picks shown on the seismic. Seismic line and geological interpretation on FTG data reproduced from Cullen (2010).

The recent shale - gas exploration successes across the US required innovative solutions for traditional geophysics, with the source and reservoir being the same target, presenting a challenge for cost effective detection. FTG's usage in such projects has been to map fault patterns both on the regional and prospect scale to assist with drill planning exercises. Mapping such structures is critical to minimise risk due to potential leakage from source and potential contamination of ground water supplied. FTG data was acquired in SE Ohio over the Utica play in 2014 for this purpose and identifies the regional strike -slip fault pattern impacting the distribution of targeted shale horizons.

Continued development

FTG development continues at pace. 3D depth imaging workflows that involve seamless combination of FTG signal with conventional gravity, ability to image geology in between survey lines, extraction of geological signal and prediction of a density field are now proven and widely revered that direct usage in challenging seismic imaging projects is truly viable.

FTG signal is transformed to a conventional gravity field and combined with legacy gravity data to produce a Full Spectrum Gravity product making it ideal for resource and play modelling concepts. Innovative analysis methodologies of the Tensor data facilitate detailed 3D imaging and accurate depiction of target struc tures. Combined with depth information accessed from seismic and other technologies, the data is transformed into a 3D density model. Results from joint work with industry leaders demonstrate the true value of FTG and represents a future direction for FTG technology.

Driving exploration with FTG

FTG fast-tracks exploration through its unique ability to not only locate basins and basement settings but also directly identify and delineate intra -basinal structuring leading to informed play model mapping. The technology is proving to be of immense value in driving exploration activity in both mature and frontier areas alike. The recent work offshore Malaysia identifies sub-carbonate basins and new play concepts leading to improved exploration in this long established producing region. Continued work along the East Africa Rift identifies new plays along Lake Malawi and detailed mapping of pertinent strike slip fault patterns impacting shale-gas exploration in the Utica of Ohio, USA.

Collective decision making by the industry's leading players, both NOCs and IOCs is now leading to ambitious plans to roll out FTG on largescale basin wide surveys that will build the new base maps that will serve to re-define known opportunities but equally important help open up new basins and play concepts.

This cost-effective exploration technology maximises opportunity helping maintain exploration strategies in the current low price environment.