Exploration Remote Sensing/GIS

Structural analysis in eastern Yemen using remote sensing data

Remote-sensing-based reconnaissance successfully correlated surface and subsurface structures and predicted source-rock distribution

Richard Harris and Dr. Mark Cooper, EnCana Corp.

Described here is the exploration technology applied in the Masilah basin of Yemen to better understand the subsurface geology. The discussion includes an introduction and project background; it then describes the structural mapping technique using remote sensing data. Illustrations describe results of the subsurface and surface structural study. The “barren” surface terrain of the area was extensively mapped with satellite images and showed strong correlation with subsurface geologic features.

Landsat images and a digital elevation model covering central and southern portions of the Masilah basin in the Republic of Yemen have been used to map surface geology and structure. Mesozoic and Cenozoic extensional features were mapped in the Hadhramaut Group from the satellite data and extrapolated to analogous subsurface structures identified on a 2D seismic grid. The structure of the outcropping Umm Er Radhuma formation was interpreted by picking control points on the base of the widespread Eocene Jeza formation.

The control points were back-interpolated onto a Digital Elevation Model (DEM) constructed from Russian topographic maps. Contouring of the gridded control points generated a structure map of the Umm Er Radhuma formation. The interpreted structure maps and faults were combined with well and seismic data to develop 2D structural cross-sections and 3D models of the study area.

The basement high underlying the key leads was elevated during Oligocene rifting, which has important implications for the deposition of Qishn-formation reservoirs. The source basins within the study area are bound by N-S trending late-Jurassic-aged faults with inconsistent surface expression. WNW-ESE Early Cretaceous, steeply dipping extensional faults bound the majority of structural traps.

WSW-ENE Oligocene structures predominate south of the rift shoulder, paralleling the spreading ridge in the Gulf of Aden. Early Cretaceous structures were reactivated and occasionally inverted during Oligocene rifting. Overall, remote-sensing-based reconnaissance has proven successful in identifying subsurface structures and predicting the distribution of source rocks.

Background

Structural geology of the petroleum basins of Yemen predominantly reflects the Mesozoic break-up of Gondwanaland. Interior basins formed during major rifting events in the Late Jurassic and Early Cretaceous. In the Tertiary, the earlier structures were overprinted and new ones were created by extension associated with the opening of the Gulf of Aden.¹ Drilling for hydrocarbons commenced in 1961 with the first commercial discovery by Hunt Oil in 1984, in the Marib Al Jawf basin in western Yemen. Activity spread to eastern Yemen with a series of discoveries made by Nexen and Total in the Masilah region through the early 1990s.

The study area is located in the southeastern portion of the Masilah basin just to the east of Nexen’s Block 14 and covers about 25,000 km², Fig. 1.² The Masilah basin is a broadly symmetrical graben oriented NW-SE, roughly parallel to and controlled by the Pre-Cambrian Najd fault trend. ³ The oldest sediments within the basin are early Jurassic in age and are preserved in small intra-basinal lows surrounded by broad Cretaceous platforms. Basin fill comprises three major sequences: the Jurassic Amran Group, the Cretaceous Tawilah Group and the Tertiary Hadhramaut Group.

Fig. 1. Structural elements and EnCana’s concessions within the Republic of Yemen. Study area is outlined by green concession boundaries. (Modified after Beydoun, 1997).2

The basin is bounded to the north by the Fartaq High and the Hadhramaut Arch, and to the South by the Mukalla high and the South Hadhramaut Arch, Fig. 1. The project was focused on exploration within concession Blocks 33, 43, 45 and 47. EnCana currently has operatorship and a working interest in Block 47 (52.5%) and Block 60 (39%).

The Mesozoic basins in Yemen formed during two separate rifting events. The first occurred during the Kimmeridgian-Berriasian, and was followed by a period of thermal subsidence and a second rifting event during the Hauterivian-Barremian.⁴ The basins appeared to open from West to East, with the Marib basin dominated by Late Jurassic and Early Cretaceous fill, followed by Masilah and Jiza’ Qamar basins, filling progressively with younger sediments. Source and reservoir rocks were deposited as both pre- and syn-rift sediments, forming traps during rifting within horsts and tilted fault blocks.

Production within the Masilah basin is predominantly from early Cretaceous Qishn formation sands, trapped in the hanging walls of rotated fault blocks, sealed by tight overlying Qishn formation limestone. The pools are sourced from Jurassic marine shales in adjacent down-faulted lows, which allowed hydrocarbon migration into the overlying and adjacent reservoirs.

The Qishn Clastic Member at Nexen’s Masilah fields reflects deposition in fluvial to tidal-flat environment, producing reservoir sandstone with porosity greater than 20% and permeability in excess of 1 D.⁵ Qishn formation sands have produced at rates up to 12,670 bopd, 29-33API oil, with little dissolved gas.³

Structural Mapping Using Remote Sensing Data

Landsat-based mapping is ideal for onshore, arid terrains with no history of glaciation. Many areas in Yemen have sparse seismic coverage and generally poor imaging quality from 2D lines. Structure hinted at by seismic mapping can be confirmed with combination of satellite imagery, Fig. 2, and surface DEM, Fig. 3. This technique provides for rapid reconnaissance of structural features, to be followed up by more detailed seismic.

Fig. 2. Raw Landsat 7 scenes over study area.

Fig. 3. Digital Elevation Model (DEM) over study area.

The Masilah area is ideal because surface structure often mimics the reservoir horizon. Seismic data shows a close correlation between the top of the Qishn formation and surface structure. Thickening of the Mesozoic strata occurs prior to deposition of the Qishn. The Masilah area has sparse surface cover and flat-lying surface strata that is extensively cut by erosion, allowing mapping of outcropping formations over large areas.

New technologies being introduced to EnCana in this project include spectral mapping, which combines Landsat images with DEM to map surface structure and geology. Spectral mapping requires using the unique combination of spectral reflectance (color), brightness and erosional texture associated with lithologic units to define spectral units bounded by recognizable surfaces on a satellite image or photograph. These units have no genetic significance until ground-truthed in the field.

The ideal terrain has not suffered glaciation and exists in an arid climate, allowing for maximum outcrop exposure. The Arabian Peninsula is one such region, and it has been mapped extensively using aerial photography and satellite images.⁶ The study area in Yemen has the added advantage of gently dipping surface stratigraphy cut extensively by drainage, allowing individual contacts between spectral units to be spatially present over the entire region.

Landsat 7 satellites have seven spectral bands ranging through the electromagnetic spectrum. Combining bands 7 (reflected infrared ‘IR’), 4 (reflected IR) and 1 (blue-green)⁷ as a RGB raster in ERMAPPER produces an image which gives the best distinction of outcropping formations in eastern Yemen, Fig. 4.

Fig. 4. Combining Landsat bands 7, 4 and 1 to create a false color composite image that exhibits best distinction between various outcropping lithologies.

Some of EnCana’s staff had field experience in Yemen and were able to help correlate the spectral units to lithostratigraphic units present throughout the field area (J. Chisholm, personal communication). The DEM is an ASCIIXYZ data file representing topography in both 100-m and 20-m postings, Fig. 3. The DEM was produced from Russian air photo stereopairs. The ASCII file was imported to ERMAPPER and converted to ‘.ers’ format for use in the creation of a 3D surface model.

Surface structure and stratigraphy was interpreted from the 3D surface model created by combining DEM with satellite images. The satellite RGB raster was draped over an intensity layer created from DEM. The combined dataset could be displayed in 3D mode , Fig. 5, to establish fault type and throw, and the stratigraphy of outcropping formations. Spectral stratigraphy of the composite Landsat image is matched to surface stratigraphy through the use of aerial photos and field work.

Fig. 5. Combining Landsat data with DEM to create a 3D surface model.

The Eocene Jeza formation is a cliff-forming unit conformably overlying Umm Er Radhuma shallow-marine carbonates, Fig. 6. The more argillaceous Jeza appears as red/brown on weathered surfaces, and the Umm Er Radhuma shows as dark purple, Fig. 4. EW striking normal faults are easily identified by appearance of younger strata at the base of fault scarps. Umm ER Radhuma carbonates were resistant to weathering and flat-lying and, thus, could be mapped consistently over the entire study area.

Fig. 6. Air photo illustrating surface topography of Umm Er Radhuma and Jeza formations.

Structure maps of the top of the outcropping Umm ER Radhuma formation, Fig. 7, and the Russ formation, were constructed as part of a structural model for the study area. The purpose of these maps was to predict subsurface structures, identify structural features for lead delineation and to rapidly focus the seismic program within Block 47. Structural data was also used as an input for cross-section construction and 3D modeling.

Fig. 7. Structure map and fault vectors constructed on top of Umm Er Radhuma formation for Block 47.

Points were selected on the Landsat raster image which represent the top contact of the Umm ER Radhuma. This formation was chosen for its widespread and cliff-forming nature, allowing for use of topography as a quality control. The number of points picked depends on structure complexity and exposure of the contact.

Surface faults were mapped as vectors and used to create a more realistic structure map. The DEM was overlayed onto the points as topographic contours to ensure that points lie along slope breaks at the base of the cliff-forming unit. The DEM grid was back-interpolated onto the pointset data to assign a Z value to each XY point.

The resultant XYZ data file represents the structure of the top of the selected formation. The resultant data file was gridded using horizontal resolution of the Landsat data as a grid increment. Contouring this grid file resulted in a structural contour map of the outcropping formation.

Groundtruthing the structural interpretation was accomplished with both field data and a 2D seismic grid. In August 2001, an EnCana field party took 54 GPS measurements at the top of the Umm Er Radhuma contact, within the central portion of Block 47.⁸ Measurements were taken to test closures over large fault-bound structures identified from the top Umm Er Radhuma contour map. An average error of 19.8 m was calculated when GPS measurements were compared to the interpreted top of the Umm Er Radhuma formation. A graph of field data vs. satellite-derived data shows that the error is consistent, and the interpretation can be corrected by bulk shifting the interpreted structure up 20 m.

Results

Several cross-sections were constructed as part of a structural model of the study area, Fig. 8. The sections were created to visualize and predict subsurface structure to identify risks associated with reservoir, seal and trap for leads within the study area. Position and orientation of each section was based on location of leads, wells and seismic lines. The sections were constructed in GEOSEC2D (Paradigm) by projecting interpreted horizons from 2D seismic with well data, topography and structure of surface formations into the line of section.

Fig. 8. Sample section (see Fig. 3) constructed by modeling surface topography, surface structure, well data and seismic interpretation.

The sections illustrate orientation and displacement of major fault-bounded structures, as well as timing of episodes of extension and inversion throughout the Mesozoic and Cenozoic. Three extensional events were identified, constraining: deposition of source rocks, formation of trapping structures and risk of seal breach due to late inversion.

Cretaceous and Oligocene faults mapped from the satellite data closely matched subsurface faults mapped from 2-D seismic; but Jurassic faults bounding source grabens were not consistently expressed as surface features. Many of the structural highs identified from surface mapping were subsequently recognized as inversion structures with important implications for reservoir deposition and seal integrity.

Sixteen leads were identified within the study area, and three were corroborated in faulted structures.  

Acknowledgment

The authors thank EnCana Corp. for permission to publish this article. They additionally thank Jeff Chisholm of Niko Resources, and Roger Fife, J. C. Chameroy, Jackie Howard, Ian Shook and Robert Gardner of Encana for their substantial contributions.

Literature Cited

¹ Csato, I., et al., “New views of the subsurface play concepts of oil exploration in Yemen,” Oil & Gas Journal, Vol. 99, No.23, 2001, pp. 36-47.

² Beydoun, Z. R., “Introduction to the revised Mesozoic stratigraphy and nomenclature for Yemen,” Marine and Petroleum Geology, Vol. 14, No. 6, 1997, pp. 617-630.

³ Bosence, B. W. J., “Mesozoic rift basins of Yemen,” Marine and Petroleum Geology, Vol. 14, No. 6, 1997, pp. 611-616.

⁴ Ellis, A. C., et al., “A tectono-stratigraphic framework for Yemen and its implications for hydrocarbon potential,” Petroleum Geoscience, Vol. 2, 1996, pp. 29-42.

⁵ Putnam, P. E., G. Kendall, and D. A. Winter, “Estuarine deposits of the Upper Qishn formation (Lower Cretaceous), Masila region, Yemen,” AAPG Bulletin, Vol. 81, No. 8, 1997, pp. 1306-1329.

⁶ Wender, L. E. and F. F. Sabins, “Geologic interpretation of satellite images, Saudi Arabia,” paper SPE 21358, 1991, pp. 213-221.

⁷ Sabins, F. F., “Digital processing of satellite images of Saudi Arabia,” paper SPE 21357, 1991, pp. 207-212.

⁸ Gavrilescu, G., “Geological survey in Block 47, Yemen,” PCE internal report, 2001.

The authors
	Richard Harris earned a BSc in geological engineering from Queen’s University in 1990, and an MSc in geology from the University of British Columbia. He previously worked East Coast Canada and the North Sea for EnCana Corp. He is presently working as a geologist for the Yemen team of the Middle East Group of EnCana.
	Dr. Mark Cooper earned a BSc (Honors) in geology from Imperial College, University of London in 1974, and a PhD in geology at the University of Bristol, 1978. Following an academic career, he worked in various international locations in oil and gas, and then joined PanCanadian in 1994 as a structural geology specialist in the Foothills District. Following positions in the North America Frontiers group, he now runs the international New Entry Assessment Team for EnCana Corp.