Abstract
Interpretation of three-dimensional (3D) seismic data from the northern part of central Saudi Arabia allowed mapping of the Permo-Carboniferous Unayzah Formation's aeolian sand deposits and fluvial channels. These types of depositional environments have been associated frequently with better reservoir-quality sandstones within Saudi Arabia. The depositional setting of the Unayzah reservoirs can be highly variable and range from glacial, to lacustrine, to meandering fluvial, to aeolian, to braided stream. Seismic amplitude data were instrumental in mapping a truncation of the upper Unayzah reservoir against the overlying Pre-Khuff Unconformity. The use of seismic attributes, such as coherence and curvature, indicated the existence of faulting and a fluvial channel network at the top of the Unayzah Formation. The integration of well and seismic amplitude data, via acoustic impedance inversion, revealed the direction of potentially higher porosity fairways within the channel system. The structural and depositional architecture depicted in this study is being appraised through drilling of an exploration well.
INTRODUCTION
Between 2000 and 2003, several high-fold 3D seismic surveys were acquired over a portion of north-central Saudi Arabia. To help understand the regional Unayzah depositional trends, these pre-stack time-migrated datasets were merged, post-stack, into a single continuous 3D volume. This merged dataset covers an area of approximately 3000 km2 and contains several oil and gas fields which produce from a number of different reservoirs. Three-dimensional pre-stack time-migrated data provide the opportunity to image and map deep stratigraphic features below 3 km (10 000 ft) with high spatial resolution (Brown 2004). These data were processed to preserve true relative amplitude so that seismic attributes and amplitude-based acoustic impedance inversion could be used for quantitative analysis. Conventional studies aimed at seismic stratigraphic plays have utilized seismic cross-sections and time slices to map depositional systems (Miall 2002). More recently, some authors have used seismic amplitudes and volumetric attributes for stratigraphic analysis (Posamentier & Kolla 2003; Neves et al. 2004). The subject reservoir is vertically and spatially heterogeneous due to the rapid variation of depositional environments and tectonic activity. The work presented in this paper shows 3D seismic interpretation, utilizing several different seismic attributes, and the integration of this interpretation with well-log data analysis over the upper Unayzah A reservoir.
GEOLOGICAL BACKGROUND
Figure 1 shows a regional-scale Landsat image of Saudi Arabia, highlighting the study area. The Permo-Carboniferous sandstones (Unayzah reservoir) of Saudi Arabia contain huge reserves of non-associated sweet natural gas. The Unayzah reservoir is a stratigraphically and climatically complex suite of sandstones comprising periglacial, hot-arid aeolian, shallow-marine estuarine and deltaic deposits (Melvin & Heine 2006). The section is punctuated by at least two unconformities. Porosity and permeability can vary significantly at all depths of burial. Nevertheless, analysis of a large number of core plug data suggests two main end-member poroperm trends. One trend is characterized mostly by coarser, relatively clean sandstones variably cemented with quartz overgrowths. A second trend consists of finer, more feldspathic sandstones dominated by diagenetic fibrous illite (Franks 2006). For any given porosity, permeability is typically an order-of-magnitude (or more) greater in the quartz trend than in the illite trend. Hence, interpretation of seismic porosity signatures or log porosity requires additional information to resolve whether the porosity is associated with sufficient permeability to be productive. In other words, cores are needed to distinguish confidently between total porosity and effective porosity. In addition fluid inclusion and/or ion microprobe data, combined with kinetic modelling, help constrain timing of quartz cementation and, by inference, of illite cementation. In some areas there is good petrographic evidence of inhibition or retardation of quartz cementation by early oil trapping, prior to gas charging. Illite and chlorite grain coats also help preserve porosity in some sandstones. In addition to burial diagenetic effects, reservoir quality is affected by proximity to palaeosols. Infiltrated (illuviated) soil clays and precipitates destroy porosity and permeability for 3–15 m (10–50 ft) below some soil zones in sandstones that otherwise would have had excellent reservoir potential. Post-glacial Permian sandstones of the Unayzah reservoir host substantial oil and gas reserves in Saudi Arabia. They are informally known as Unayzah A and Basal Khuff Clastics (seismic marker referred to as BKDC in this paper). Facies analysis of these deposits revealed the presence of playa, sabkha, flood plain, ephemeral stream and sheet flood, and aeolian facies. Furthermore, a wide variety of palaeosols, reflecting arid and semi-arid climates, exist throughout the reservoir. An eastward progradational terminal fan-bajada system is recognized. The system was repeatedly reworked by easterly winds that developed widespread aeolian dune and sand sheet deposits. Rapid marine transgression also recycled the topmost part of the terminal fluvial system and deposited thin restricted marine facies at the top of the reservoir. Evolution of the terminal fluvial system is marked by the development of three major tectonostratigraphic sequences that are bounded by unconformities and correlative conformities. Deposition of the lower sequence was marked by a relative tectonic quiescence, uniform subsidence, overfilled basin conditions, and consistent aggradation and progradation of a transverse terminal fluvial system into extensive playa lakes in the east. The middle sequence experienced faulting, uplift of proximal areas and development of extensive sequence boundaries in the west, subsidence and formation of correlative conformities in the east, widespread development of local playa lakes, sediment bypass and forced progradation of the terminal fluvial system. The upper sequence recorded intense faulting and structural inversions, under-filled basin conditions, overall retrogradation of fluvial systems and consequent marine transgression. Minor axial fluvial systems, in addition to the main transverse one, were developed during this period. Widespread palaeosols with distinct catenas were also formed. The latter is a sequence of soils of similar age, which were derived from the same parent material and occurred under similar climatic conditions. In central Saudi Arabia, these catenas may display different characteristics due to variation in relief and in drainage. They reflect, respectively, seasonally wet climates and contemporaneous structural topography. The wet climate, combined with active tectonics, resulted in more competent flows, development of structurally controlled arroyos (desert stream beds, usually dry but wet after rain) and enhanced primary coarse sediment supply. The Unayzah sequences record a gradual upward change from predominantly fluvial to widespread aeolian and then back to prevailing fluvial systems. This trend reflects transitions from semi-arid to arid to semi-arid climates. It is inferred that these low-order climatic shifts coincided with the northward drift of NE Gondwana into the trade wind and lower latitude climatic belts in addition to progressive transition towards greenhouse conditions. High frequency wet–dry–wet cycles are superimposed on the low-order climatic shifts. They reflect autogenic processes, local subsidence events and climatic fluctuations. The latter are related to orbital perturbations. Progradation of individual terminal fan lobes was punctuated cyclically by periods of soil formation and wind reworking. Coincidence of arid periods of the medial low- and high-order cycles developed an extensive dry aeolian system that corresponds to the most arid climate. During this peak aridity period, thick and dry aeolian dunes preferentially accumulated in syn-depositional structural lows with faults facing easterly winds. Tectonics controlled the large-scale stratigraphic architecture of the Unayzah terminal fluvial and associated aeolian systems. It also forced the localization of the main facies belts. In contrast, climate controlled temporal facies changes and greatly affected sediment supply and its quality (Franks 2006).
Location map showing a true colour Landsat image of Saudi Arabia. The study area (400 km2) is located within the red rectangle.
The upper Permian Unayzah A reservoir in the study area in central Saudi Arabia consists of sandstone and siltstone units with a maximum total thickness of about 110 m (350 ft) in the study area. The upper part of the Unayzah formation consists of fluvial, aeolian dune, and interdune deposits in the four wells studied. Reservoir quality of the dune facies is generally excellent, with the quality deteriorating and becoming silty in the interdune facies. The better quality reservoir facies (aeolian and fluvial) is referred to in this paper as the Unayzah A. Figure 2 shows a stratigraphic cross-section between the four wells, datumed at the Pre-Khuff Unconformity. At well W1, 30 m (100 ft) of relatively porous (above 5%) sand was encountered, but it was rather tight, with average permeability below 10 mD. Well W2 drilled 60 m (200 ft) of quality reservoir sands. No Unayzah reservoir was found at well W3. W4 also had a 60 m (200 ft) thick sand with porosity around 10%. The source rock for the Unayzah reservoirs is the hot shale within the Silurian Qusaiba Member (seismic marker QUSB). A primary objective of the exploration programme in the study area is to determine zones of high porosity within the different sandstone facies and where these zones pinchout from well W1 or W2 towards W3.
Schematic stratigraphic cross-section across key wells (W1, W2, W3 and W4) showing the pinchout of the gross Unayzah reservoir towards W3. The source rock is the underlying hot shale. Log curves in each well panel (from left to right) are: (1) gamma-ray (green) and caliper (red curve), (2) lithology (yellow is sandstone, brown is silt/shale, green is carbonate) and (3) deep induction (dark blue), density (black), neutron porosity (red), and monopole sonic (black rightmost curve). The proposed exploration well is indicated between wells W1 and W3. The Unayzah net sandstone thickness in W1 is approximately 30 m (100 ft).
SEISMIC DATA INTERPRETATION
The seismic time interpretation was made using cross-sections, time slices and horizon slices. Several geological horizons were mapped both above and below the Unayzah reservoir of interest. Figure 3 show the zero-offset, acoustic, seismic forward modelling using sonic and density logs from wells W1, W3 and W4. Well W2 was excluded because of its close proximity to the fault zone, which degraded the quality of the seismic data substantially. These well-ties were adopted to characterize the seismic signatures (seismic facies) of the reservoir interval. Figure 3a shows that at well W1 a 30 m (100 ft), tight (low permeability) Unayzah sandstone is characterized by a seismic trough (negative amplitude) beneath a large seismic peak (positive amplitude). Here, a seismic polarity convention is adopted in which a decrease in acoustic impedance results in a positive, black seismic peak. Figure 3b indicates that at well W3, where no Unayzah reservoir is present, the seismic signature is a large seismic peak. The synthetic in Figure 3c, at well W4, displays a weak seismic trough underlying a seismic peak, a signature which, from regional experience, is associated with the presence of porous reservoir and is expressed more strongly in the real seismic traces to the right of the figure. In the case of W4, the 60 m (200 ft) reservoir interval is characterized by a velocity gradient in the sonic, terminating in a weak impedance contrast at base reservoir, which results in a peak over a trough seismic signature on the synthetic, indicating the presence of the reservoir. Figure 4 shows the extracted seismic root mean square (rms.) amplitude in a 20 ms window below the top of the reservoir. The red–yellow colours indicate large amplitude values, while the blue ‘colder’ colours represent small values. Well W1 with its thick and less porous reservoir is located in a relatively higher amplitude area compared to well W3. Well W4 (thick and porous reservoir) is also in a relatively large seismic amplitude (green) compared to W3 (dark blue). The proposed well location (red circle) is located in a higher amplitude area and has been interpreted and modelled to be associated with a thick and porous clastic reservoir deposited in a classic ox-bow sedimentary setting (Fig. 4 inset). The seismic interpretation of an incipient ox-bow lake is based upon its characteristic U-shape (dotted brown lines in Fig. 4) and the imaging of a neighbouring meandering network (dotted black lines in Fig. 4) as seismically observed. Note that the meandering features (black dotted lines) were better imaged by other seismic attributes such as coherence (or semblance) and curvature. Seismic forward modelling also indicated that seismic tuning (thin-layer effect) could not alone explain an order of magnitude increase in seismic amplitude observed at the proposed well location. The grey zones running approximately north–south are major reverse faults which seem to exert some degree of control on the seismic amplitudes. Figure 5a is a seismic traverse from south to north across all four wells showing several reverse and normal faults (sub-vertical dark blue solid lines). It also shows the high-amplitude seismic anomaly (white event between BKDC and QUSB/BQSBmarkers in Fig. 5a) which correlates with the porous Unayzah reservoir updip from well W4. The proposed exploration well targets this bright seismic amplitude anomaly. Figure 5b shows the wildcat location is targeting the anticipated presence of a more porous (low impedance) reservoir than the area around well W3 where the Unayzah A formation was found to be absent (coincident markers in Fig. 5a). Figure 6 shows a NW–SE seismic traverse across wells W2 and W3. Note the interpretation of both normal and reverse near-vertical faults (thin and thick red lines, respectively). In order to aid and hasten the fault mapping effort, several seismic attributes calibrated and validated by well data were utilized. Recently, several authors (e.g. Al-Dossary & Marfurt 2006; Chopra & Marfurt 2007) have illustrated the added value of seismic volumetric attributes for fault and fracture mapping. Multi-trace geometric attributes, such as coherence (also called semblance) and curvature facilitate the identification of seismic lineaments by eliminating the need for accurate picking of irregular horizons due to noisy seismic data or where no laterally consistent impedance contrast exists. Coherence and curvature are complementary seismic attributes: while the former attribute measures waveform similarity between seismic traces, the latter attribute measures the reflector morphology. Figure 7 shows time slices of coherency and curvature attributes. Several seismic lineaments and discontinuities can be observed clearly and were interpreted as faults and channels at the Unayzah level. The white circles show the location of the existing wells and the proposed well is in red. Isopach and acoustic impedance mapping using seismic and well-log information were used to define two distinct sand fairways.
Seismic well-ties for (a) W1, (b) W3 and (c) W4. (a) Seismic facies for the 30 m (100 ft) tight reservoir interval (highlighted in yellow) is a trough below a seismic peak. (b) Seismic facies for non-reservoir interval (between BKDC and QUSB markers) is a seismic peak. (c) Seismic facies for 60 m (200 ft) porous reservoir interval (highlighted in yellow) is a weak seismic trough below a seismic peak, caused by the velocity gradient in the sonic and the low impedance contrast at the base of the reservoir (the reservoir signature is expressed more strongly in the field seismic data to the right). Correlation coefficients between synthetic (left seismic panel) and field seismic data (rightmost panel) data are (a) 0.71, (b) 0.67 and (c) 0.61.
Seismic rms. amplitude map extracted near top of the reservoir. Bright colours (red/yellow) are high amplitude values, while cold colours (light/dark blue) are low values. Both W1 and W4 fall into comparable rms. seismic amplitude (green/light blue) ranges. Well W3, where no reservoir was encountered, is in a low amplitude region (blue colour) highlighted by the dashed area. The proposed well location is the red solid circle. The grey zones running roughly north–south are seismically mapped major reverse faults.
Two displays of a traverse through three wells and the proposed well location. Note that the bright seismic amplitude anomaly seen between the marker horizons in (a) is generated by low impedance (relatively high porosity) at the proposed well location observed in (b). The proposed well (dashed white vertical straight line) targets the large seismic amplitude anomaly observed at the reservoir level. (b) The observed seismic amplitude anomaly corresponds to a low impedance (porous) interval at the proposed well location. Sub-vertical solid blue lines in (a) are interpreted faults. In (b) two portions of the low impedance target reservoir are enclosed by red polygons. Low impedance (high porosity) is represented by red colours while high impedance (low porosity) is blue.
Seismic traverse across well W2 and W3 showing the near-vertical reverse faults (in thick red) and a normal fault (thin red). Note the large vertical throw between W2 and W3. Reservoir interval is between geological markers BKDC (purple) and HU (blue). The interpreted reservoir pinchout can be observed towards W3 (coming from the right of the figure).
Coherency (left) and curvature (right) slices. Volumetric seismic attributes revealing the existence of several seismic lineaments and discontinuities, which are interpreted to be faults (green arrow) and channels (red arrow) at the Unayzah A level. White and red circles indicate existing and proposed wells, respectively.
CONCLUSIONS
Interpretation of a large 3D seismic dataset from central Saudi Arabia allowed detailed mapping of a network of fluvial channels typically associated with better reservoir-quality sandstones. Understanding the relationship between the seismic forward modelling from the well-logs was important for characterizing the seismic signature of the reservoir. Seismic amplitude, curvature, coherence and acoustic impedance data were integrated with existing well data to map potentially higher total porosity fairways within the reservoir, and a truncation of the reservoir against the Pre-Khuff Unconformity. This interpretation guided selection of an exploration well location which is being drilled presently to test the interpretation.
Acknowledgements
The authors thank Greg Douglass and Garth Jahraus for reviewing this abstract. Thanks also go to Galet Aida, Khalil Sulami and Roger Price for their guidance and for supporting this work, as well as to Saudi Aramco management for permission to publish this study.
- 2008 EAGE/Geological Society of London
References
Interpretation and mapping of a stratigraphic play using 3D seismic data and attributes in central Saudi Arabia
