Small Fault Blocks Research Articles

Effective Water Injection Development Test for Rolling Outward Expansion of Low-grade Reservoir

The development of ultra-low permeability reservoirs in the oilfield mainly involves a large proportion of ultra-low permeability and it is difficult to recover reserves because of scattered distribution, low reserve abundance, poor reservoir physical property, and great development difficulty as well as low production intensity, rapid production decline, and low stable production of oil wells. In order to speed up the development of undeveloped reserves and explore the effective technical approach to solve problems in low permeability and difficulty in recovering reservoirs, we carried out the effective water injection development test in the B1 test area. In this paper, in-depth research has been carried out in reservoir geological understanding, development well pattern design optimization, and advanced water injection technology research. Finally, a set of effective production technologies for low abundance reserves in complex geological conditions are formed, effective production in B1 Block that was hard to recover reserves is realized, the drilling success rate is improved greatly, and well pattern design optimization is achieved, advanced water injection efficient development technologies for low abundance reserves blocks are also developed. The demand for rolling production can be gradually extended to other oilfields, which has certain reference significance for the development practice of peripheral oilfields and similar small complex fault block oilfields.

Study on water flooding law in high water cut stage of complex fault block reservoir

In recent years, with the progress of science and technology and the development of times, it has brought great opportunities for the development and protection of Tibetan resources in our country. Many reservoirs are located in very complex fault blocks with small fault blocks and many faults, coupled with strong heterogeneous media. After years of waterflooding development, they have entered the high water cut oil recovery stage. In view of this, this paper will focus on the analysis of the water drive law in the high water cut stage of complex fault block reservoirs, and carry out specific strategy optimization for increasing production, in order to promote the exploration of the law of high water cut area in complex fault block reservoirs.

Diffraction separation and imaging based on double sparse transforms

Reflection imaging results generally reveal large-scale continuous geological information, and it is difficult to identify small-scale geological bodies such as breakpoints, pinch points, small fault blocks, caves, and fractures, etc. Diffraction imaging is an important method to identify small-scale geological bodies and it has higher resolution than reflection imaging. In the common-offset domain, reflections are mostly expressed as smooth linear events, whereas diffractions are characterized by hyperbolic events. This paper proposes a diffraction extraction method based on double sparse transforms. The linear events can be sparsely expressed by the high-resolution linear Radon transform, and the curved events can be sparsely expressed by the Curvelet transform. A sparse inversion model is built and the alternating direction method is used to solve the inversion model. Simulation data and field data experimental results proved that the diffractions extraction method based on double sparse transforms can effectively improve the imaging quality of faults and other small-scale geological bodies.

Ordovician Fauna in a Small Fault Block on the Yarrol Fault, South of Calliope, Central Queensland

Ordovician Fauna in a Small Fault Block on the Yarrol Fault, South of Calliope, Central Queensland

Effects of the interaction of tectonics, eustasy, climate, and minerals on the sedimentary evolution of early-middle Jurassic in the Hongshanzui region in Junggar Basin

Majority of the oilfields in the Hongshanzui region are small and complex fault block reservoirs, and the evolution of the sedimentary system under the sequence stratigrapic background has been a controversial topic. Six middle-term base-level cycles were divided, and eight types of lithofacies assemblages were recognized based on the stacking patterns and volumetric partitioning according to the core, well logs, and seismic data correlated with the field investigation of outcrops. Furthermore, the evolution of the sedimentary system under the sequence stratigraphic framework was clarified. Among the six middle-term base-levels, MSC1 belongs to the retrogradational system tract, and the sediment distribution exhibits the characteristics of “large plain and small front.” MSC2 belongs to the transgression system tract, and the sediment distribution exhibits has the characteristics of “small plain and large front.” In the case of MSC5 and MSC6, the sediment distribution exhibits the characteristics of “dominant channel and repeated channel-switching.” Further, the controlling factors of tectonic, base level, climate, and minerals are discussed.This research has been applied to the exploration and development of the Hongshanzui oilfield, and a certain benefit has been achieved.

Analysis on the Development Status and Countermeasures of Fault Block Reservoir in Middle and Late Stage

China is a country with rich oil reserves. With the development of the reservoir, the proportion of fault block reservoir (hereinafter referred to as FBR) discovery increases gradually. At present, FBR has occupied a considerable proportion of China’s oil reserves. Zhongyuan Oilfield is mainly composed of complex FBR. FBR has unique characteristics, such as small fault block and complex geological structure, which will greatly increase the difficulty of development in the middle and late development. Therefore, we must comprehensively adjust and tap the potential of FBR, which will achieve long-term stable production in the middle and late stage of fault block oilfield. Therefore, based on the characteristics and classification of FBR, this paper analyzes the development status in the middle and late stage. Then, according to the characteristics of fault block oilfield in the middle and late stage, this paper puts forward some comprehensive adjustment technical measures, which will better adapt to the reasonable development mode of FBR in the middle and late stage.

Research on Well-seismic Characterization of Multi-layer Sandstone Reservoir

Multi-layer sandstone reservoirs are one of the important types of A dilute oil reservoirs. They have reached the middle and later stages of development, with the following characteristics: controlled by multiple multi-level faults, many faults, small fault blocks, small fault block reservoir area; affected by sedimentation, the distribution of sand bodies is discontinuous and the connectivity is poor, resulting in serious reservoir heterogeneity; after long-term water injection scouring, the reservoir properties and oil-water properties have changed, and the remaining oil distribution is complicated. These characteristics determine that the research focus of this type of reservoir is to carry out fine characterization of reservoir well-seismic joints, establish the isochronous stratigraphic framework of the reservoir unit, analyze the characteristics of small faults and microstructures in the reservoir, and describe various sand bodies between wells. The internal heterogeneity and the three-dimensional spatial distribution regularity of the oilfield are used to establish a fine three-dimensional reservoir geological model, which provides strong technical support for revealing the spatial distribution characteristics and laws of the remaining oil.

An Integrated Method of Data Mining and Flow Unit Identification for Typical Low Permeability Reservoir Prediction

With the development of oilfield exploration and mining, the research on continental oil and gas reservoirs has been gradually refined, and the exploration target of offshore reservoir has also entered the hot studystage of small sand bodies, small fault blocks, complex structures, low permeability and various heterogeneous geological bodies. Thus, the marine oil and gas development will inevitably enter thecomplicated reservoir stage; meanwhile the corresponding assessment technologies, engineering measures andexploration method should be designed delicately. Studying on hydraulic flow unit of low permeability reservoir of offshore oilfield has practical significance for connectivity degree and remaining oil distribution. An integrated method which contains the data mining and flow unit identification part was used on the flow unit prediction of low permeability reservoir; the predicted results were compared with mature commercial system results for verifying its application. This strategy is successfully applied to increase the accuracy by choosing the outstanding prediction result. Excellent computing system could provide more accurate geological information for reservoir characterization.

Controlling factors and accumulation model of hydrocarbon reservoirs in the Upper Cretaceous Yogou Formation, Koulele Area, Termit Basin, Niger

Based on the sedimentary and tectonic background of the Termit Basin, this paper focuses on the Upper Cretaceous Yogou Formation and uses organic geochemistry, logging, oil testing and seismic data to analyze the primary control factors of the hydrocarbon accumulation and establish corresponding model in order to predict favorable exploration target zones of hydrocarbon reservoirs. This study demonstrates that the Upper Cretaceous Yogou Formation is a self-generation and self-accumulation type reservoir. The Yogou Formation hydrocarbon reservoirs in the Koulele area are controlled by four factors: (1) the source rock is controlled by a wide range of YS1-YS2 marine shale, (2) the sandstone reservoir is controlled by the YS3 underwater distributary channel and storm dunes, (3) migration of hydrocarbons is controlled by faults and the regional monocline structure, and (4) the accumulation of hydrocarbons is controlled by lateral seal. The structures in the western Koulele area are primarily reverse fault-blocks with large throws, and the structures in the east are dominantly fault-blocks with small throws (co-rotating and reverse) and a fault-nose. In the western Koulele area, where the facies are dominated by storm dunes on a larger scale, it is easier to form lithologic reservoirs of sandstone lens. In the eastern Koulele area, high-quality channel sandstone reservoirs, fault-blocks with small throws, and the monocline structure benefit for the formation of updip pinch out lithologic traps, fault lithologic reservoirs and fault-nose structural reservoirs. Future exploration targets should be focused in the western storm dunes zone and eastern distributary channel sand zone with small fault-blocks.

老井复查技术在小断块油田的应用研究

老井复查技术在小断块油田的应用研究

小断块油藏低级序断层识别与构造建模研究

本文以贝尔凹陷小断块油藏构造建模实践为例，针对低级序断层存在的断点组合困难、延伸距离短和落差小而难以发现和描述等问题，利用三维地震解释技术、petrel及RMS软件，对小断块油藏低级序断层特征进行深入研究，定量识别和描述了低级序断层，建立了小断块油藏精细构造模型，指导了油田中后期剩余油挖潜，且取得了好效果。 Taking structure modeling practice as an example of Beier depression small fault block reservoirs, we aim at the problems such as the existing difficult breakpoint combination, short extension distance, small falling which are difficult to find and describe and so on in the low-order faults. By using 3D seismic interpretation technology, petrel and RMS software, we further study the characteristics of small fault block reservoirs of low-order faults, then quantitatively identify and describe the low-order faults, establish its fine structure model, guide the dig of remaining oil in the middle-later period of oilfiled development and achieve good results.

Seismic Imaging of Small Faults in Coal Seams by Prestack Time Migration

In order to improve imaging precision of small structures and small fault blocks of coal seams, the prestack time migration method is used for imaging. Preserved amplitude processing (PAP) is applied to prestack gather firstly after geological data and original seismic data of the exploration area are fully understood. Initial root mean square velocity field is established through the method of picking up root mean square velocity on CRP gather. Then, a precise root mean square velocity model is created after continuous iteration and modification. Meanwhile, appropriate algorithm and migration parameters are selected during the migration process. Finally, the imaging of small fault blocks and small faults in the prestack time migration section is clear and migration is highly coinciding with the case disclosed by boreholes. It’s proved that prestack time migration is especially suitable for processing 3D seismic data of small faults and small fault blocks in coal seams with complicated geological conditions.

The 3-D geometry of detachment faulting at mid-ocean ridges

Seismic images of Cretaceous slow spreading crust from the eastern Central Atlantic provide new constraints on the process of seafloor spreading and the importance of detachment faulting. The seismic depth sections image detachment faults that appear to have exhumed footwall massifs of similar geometry to massifs of plutonic and mantle rocks mapped at the present Mid-Atlantic Ridge. The detachments are consistent with the structure and microearthquakes of the Mid-Atlantic Ridge at 26°N, with the footwall rotation inferred from paleomagnetic data and with numerical modeling of oceanic detachments. Other seismically imaged detachments have similar dimensions and geometry, but are covered by a layer of small fault blocks. The detachment types differ in whether or not the fault locks up in the subsurface, probably controlled by the fault strength, the elastic thickness, and whether the exhumed footwall is partly covered by basalts. Toward the segment middle, decreasing mantle serpentinization, decreasing elastic thickness, and thicker median valley basalts all increase the likelihood that the fault locks up, and a new fault propagates upwards from the still active root zone, transferring a slice of the hanging wall to the footwall, to be rafted with the footwall out of the median valley. As a result an oceanic detachment fault, exhuming the footwall at a segment end to form an oceanic core complex, may disappear laterally beneath rafted blocks; detachment faulting may be more widespread at slow spreading ridges than interpreted from seafloor mapping.

Rhodolith transport and immobilization on a volcanically active rocky shore: Middle Miocene at Cabeço das Laranjas on Ilhéu de Cima (Madeira Archipelago, Portugal)

Extraordinary deposits of fossil rhodoliths occur at the Cabeço das Laranjas (Portuguese~Hill of the Oranges) in a small fault block at the northwest end of Ilhéu de Cima off Porto Santo in the Madeira Archipelago. Stratigraphic repetitions of densely packed rhodolith beds up to 2.6m thick are associated with a receding rocky shoreline, and are interpreted as the result of hurricanes. The initial storm deposit sits unconformably on basalt and eroded basalt boulders associated with tuff and volcaniclastic breccia. Approximately 90,000 rhodoliths of Middle Miocene age (14–15Ma) are exposed on the upper surface of the initial deposit over a 450-m2 shelf exhumed from the hill's southeast side. Ranging in diameter from ≤3cm to 20cm, many of the rhodoliths generated by crustose coralline red algae are now iron stained and resemble a mass of oranges in gross appearance. Sea stacks and large boulders rise through the thick basal rhodolith bed to form small catchment areas that held the deposit in place after the storm's passage. The succeeding rhodolith deposits are variably separated by layers with mixed carbonate and volcanic sand, pure volcanic lapilli, and volcaniclastic tephra mixed with tuff showing swaley cross-stratification. Three out of four rhodolith beds are truncated against the flank of the adjoining rocky shore. Only the youngest (fourth) rhodolith layer is fully exposed around the perimeter of the hill and can be shown to cross a basalt barrier that is traceable for 70m in cross section as an erosional ramp dipping from 6° to 8° southeast. The entire fossil-rich sequence is capped by a basalt flow showing columnar disjunction. Based on thin-section analysis, three genera of coralline red algae are recognized in the basal rhodolith deposit: Sporolithon, Lithothamnion, and Neogoniolithon. Associated biodiversity is low, represented by 16 kinds of marine invertebrates dominated by encrustations and borings on the rhodoliths and very few free body fossils. The Madeira region of the North Atlantic may have been susceptible to major cyclonic storms immediately after the Middle Miocene Climate Optimum, when a northward shift of the Inter-tropical Convergence Zone was stimulated by a steeper temperature gradient in the southern hemisphere related to expansion of continental glaciers on Antarctica.

Post-collisional deformation of the Anatolides and motion of the Arabian indenter: A paleomagnetic analysis

In the Anatolides of Turkey the neotectonic (post collisional) phase of deformation embraces the period since final closure of the southern arm of Neotethys in mid-Miocene times. The Arabian Shield indenter has continued to deform into the weak Anatolian accretionary collage resulting from subduction of this ocean by a combination of differential movement relative to the African Plate and counterclockwise (CCW) rotation. Much of resulting deformation has been accommodated by slip along major transforms comprising the North Anatolian Fault Zone (NAFZ), the East Anatolian Fault Zone (EAFZ) and the northward extension of the Dead Sea Fault Zone (DSFZ) but has also been distributed as differential block rotations through the zone of weak crust in between. Facets of this deformation comprise crustal thickening and uplift to produce the Anatolian Plateau, establishment of transform faults and tectonic escape as Arabia has continued to impinge into the Anatolian collage. Paleomagnetic analysis of this deformation is facilitated by the widespread distribution of neotectonic volcanism and graben infills, and rotations relative to the Eurasian reference frame are recognised on two scales. Rapid rotation (up to 5°/10,000 years) of small fault blocks is identified between master faults along the intracontinental transforms but deformation does not extend away from these zones and shows that seismogenic upper crust is decoupled from a lower continental lithosphere undergoing continuum deformation. The broad area of weak accreted crust between the transforms is dissected into large fault blocks which exhibit much lower rotation rates (mostly < 1°/100,000 years) that vary systematically across the Anatolides. Large CCW rotations near the Arabian indenter diminish westwards to become zero then CW near the limit of tectonic escape in western Turkey. The view that the collage has rotated anticlockwise as a single plate, either uniformly or episodically, during the Neotectonic era is refuted. Instead, deformation has been distributed and differential as the collage has adapted to changing tectonic regimes. Crustal extrusion to the west and south has expanded the curvature of the Tauride Arc and combined with retreat of the Hellenic Arc to produce the extensional horst and graben province in western Turkey. A challenge of present work is to resolve the temporal framework of tectonic rotation. Evidence from the Cappadocian volcanic province and Sivas Basin in central Anatolia indicates that rotation has been concentrated within the last 2–3 million years of the neotectonic era and therefore correlates with establishment of the intracontinental transform framework. Thus we recognise two phases to the evolution of this sector of the orogen: the first embraces crustal thickening and uplift with initiation defined specifically by transition from marine to terrestrial deposition in the Serravallian at ~12 Ma, and the second embraces crustal extrusion to the west motivated by continuing northward movement of Arabia and roll back on the Hellenic Arc since late Pliocene times. Latitudinal motions detected by paleomagnetism are close to confidence limits and consistent with small northward motion of the Anatolides since Eocene times including up to a few hundred km of closure linked to crustal thickening since the demise of NeoTethys. The driving motion from the Arabian indenter can be partially resolved from the widespread basaltic volcanism that occurred along the periphery of the Arabian Shield at 12–18 Ma during final stages of collision along the Bitlis Suture. This defines CCW rotation of 13–21° with respect to Eurasia. An average CCW rotation of 0.9°/Myr since closure of the Bitlis Suture in mid-Miocene times is unlikely to have been uniform because it has been linked to three adjoining interactions namely episodic opening of the Red Sea, a transition from crustal thickening to tectonic escape in the Anatolian collage and variable rates of strike slip motion on the DSFZ.

Exploration strategies for complex fault block reservoirs in the Subei basin

The geological background of the Subei basin is that of small relief subsidence, low geothermal gradient, multi-sedimentary hiatuses, intense reconstruction of the basin, frequent magmatic activity, and a unique combination of source rock and reservoir. This geological background resulted in characteristics such as many small fault blocks, multiple oil-bearing formations, scattered oil distribution, mini- to small-sized reservoirs, and difficulties in exploration. Aimed at such characteristics, an effective exploration strategy was adopted, and the oil reserves, production and economic benefits of the Jiangsu oilfield were significantly increased. This exploration strategy included understanding the hydrocarbon generation mechanism of source rocks, progressive evaluation of oil resources, comprehensive research on the faulted systems, the distribution of oil reservoirs and their controlling factors. The techniques used included integration of acquisition, processing and interpretation with 3-D seismics as the core technology, trap description and evaluation, directional drilling and cluster drilling, integration of cuttings logging, gas chromatographic logging and geochemical logging, and integration of early reservoir description and progressive exploration and development. This strategy could be guidance for other complex fault blocks.

Extreme crustal thinning in the south Porcupine Basin and the nature of the Porcupine Median High: implications for the formation of non-volcanic rifted margins

The southern Porcupine Basin is characterized by axial stretching factors that are greater than six and typical of rifted margins. As such, the basin can be regarded as a natural laboratory to investigate the evolution and symmetry of rifting leading towards continental separation and breakup. A bright reflection (here named P) cuts down to the west from the base of the sedimentary section, is overlain by small fault blocks and appears to represent a detachment fault. P may in part follow the top of partially serpentinized mantle: this interpretation is consistent with gravity modelling, and with numerical models of crustal embrittlement and mantle serpentinization during extension. Furthermore, P closely resembles the S reflection west of Iberia, where such serpentinites are well documented. Although overall the basin remains symmetrical, the consistent westward structural dip of the detachment implies that, at high stretching factors, extension became asymmetric. Farther south, the ‘Porcupine Median High’, appearing lens-shaped in cross-section, overlies the tilted fault blocks and is onlapped by postrift sediment. Despite no evidence for synrift magmatism, this high has previously been interpreted as a basaltic structure. However, it develops above the line of intersection of the crust–mantle boundary with the P detachment, and hence may be related to the spatial limit of serpentinization. The median high may represent a serpentinite mud volcano or diapir; we suggest that such structures produce the serpentinite breccias found within the rifted continent–ocean transition of nonvolcanic margins.

Siliciclastic shoreline to growth‐faulted, turbiditic sub‐basins: The Proterozoic River Supersequence of the upper McNamara Group on the Lawn Hill Platform, northern Australia

The River Supersequence represents a 2nd‐order accommodation cycle of approximately 15 million years duration in the Isa Superbasin. The River Supersequence comprises eight 3rd‐order sequences that are well exposed on the central Lawn Hill Platform. They are intersected in drillholes and imaged by reflection seismic on the northern Lawn Hill Platform and crop out in the McArthur Basin of the Northern Territory. South of the Murphy Inlier the supersequence forms two south‐thickening depositional wedges on the Lawn Hill Platform. The northern wedge extends from the Murphy Inlier to the Elizabeth Creek Fault Zone and the southern wedge extends from Mt Caroline to the area south of Riversleigh Station. On the central Lawn Hill Platform the River Supersequence attains a maximum thickness of 3300 m. Facies are dominantly fine‐grained siliciclastics, but the lower part comprises a mixed carbonate‐siliciclastic succession. Interspersed within fine‐grained facies are sharp‐based sandstone and conglomeratic intervals interpreted as lowstand deposits. Such lowstand deposits represent a wide range of depositional systems and palaeoenvironments including fluvial channels, shallow‐marine shoreface settings, and deeper marine turbidites and sand‐rich submarine fans. Associated transgressive and highstand deposits comprise siltstone and shale deposited below storm wave‐base in relatively quiet, deep‐water settings similar to those found in a mid‐ to outer‐shelf setting. Seismic analysis shows significant fault offsets and thickness changes within the overall wedge geometry. Abrupt thickness changes across faults over small horizontal distances are documented at both the seismic‐ and outcrop‐scales. Synsedimentary fault movement, particularly along steeply north‐dipping, largely northeast‐trending normal faults, partitioned the depositional system into local sub‐basins. On the central Lawn Hill Platform, the nature of facies and their thickness change markedly within small fault blocks. Tilting and uplift of fault blocks affected accommodation cycles in these areas. Erosion and growth of fine‐grained parts of the section is localised within fault‐bounded depocentres. There are at least three stratigraphic levels within the River Supersequence associated with base‐metal mineralisation. Of the seven supersequences in the Isa Superbasin, the River Supersequence encompasses arguably the most dynamic period of basin partitioning, syndepositional faulting, facies change and associated Zn–Pb–Ag mineralisation.

The Ardèche palaeomargin of the South-East Basin of France: Mesozoic evolution of a part of the Tethyan continental margin (Géologie Profonde de la France programme)

This paper incorporates the results of seismic and gravity investigations, borehole data and studies of sedimentology, biostratigraphy, sequence stratigraphy and structural geology, carried out in the framework of the Deep Geology of France programme. The main purpose of the paper is to define the structure and to reconstruct the evolution of the Ardèche palaeomargin of the South-East Basin of France from the Stephanian to the end of the Mesozoic. The acquisition and processing of three seismic reflection profiles, the drilling of two scientific boreholes (BA1 and MM1), and subsequent structural modelling, with the use of cross-section balancing techniques applied to both the depth-migrated seismic and geological sections, provide new geometrical constraints which lead to the following hypothesis of the geodynamic evolution of the margin: 1. (1) Late Palaeozoic extension with the development of high-angle basement faults, asymmetrical Permian half-grabens, (pre-extension) sedimentary cover. 2. (2) During the Middle Triassic, a pre-rift stage with the creation of small fault blocks and, in the Late Triassic, the formation of an E-W flexure. 3. (3) End-Lias (Early Jurassic) to Middle Jurassic formation of a major detachment in the Carboniferous coal measures, leading to complete decoupling of the Mesozoic sedimentary cover from the basement. This episode of extension, coeval with the opening of Tethys, can be related to a phase of episodic rifting between the late Sinemurian and the late Bathonian (i.e. during 30 Ma). 4. (4) Late Jurassic to Early Cretaceous thermal subsidence, with regional downwarping of the basement towards the east. 5. (5) After the Late Cretaceous (the period of maximum burial), Pyrenean and Alpine deformation in the south and east, causing progressive uplift of the Ardeche palaeomargin, as much as two kilometres of Mesozoic deposits subsequently being eroded from the section during the Tertiary.

Impact of Grid Selection on Reservoir Simulation

Summary This paper presents a field case study of a faulted, multilayer carbonate reservoir to quantify the impact of grid selection on the reservoir history match. Comparison of the history match using both orthogonal and nonorthogonal grids revealed that the simple five-point scheme can lead to significant error in the history match and in the different representations of the reservoir. Introduction A simulation study of the Thamama reservoir, a faulted carbonate reservoir in Abu Dhabi, investigated flow communication across faults and evaluated development options for the field. The field is an elongated anticline that covers 62 sq miles. Geophysical and geological data and early reservoir performance indicated that the field was separated into several fault blocks by partially sealing and nonsealing faults. Two full-field simulation models were constructed that used two different approaches to represent faults:an orthogonal (regular) grid model with faults defined at block boundaries anda nonorthogonal (irregular) grid model with block boundaries defined at fault traces. The first approach results in a greater number of cells and presents difficulties in representing the fault geometry properly. However, finite-difference techniques can be used for numerical calculations. The second approach uses fewer gridblocks and provides an accurate description of the fault pattern, but transmissibility calculations could lead to significant errors because the grid is locally distorted. To quantify the impact of grid selection on the reservoir history match, both models were used and the results compared in the simulation study. History matching was performed first with the orthogonal grid model. Then, a history match using the same matching parameters was conducted on the nonorthogonal grid model. Background The Thamama reservoir comprises three zones, Zones A, B, and C. Characterized by high faulting and low permeability, this carbonate reservoir contains about 3.6 billion STB of oil in place (OIP). Zones B and C, which contain about 80% of the total OIP, have been the focus of major development since the field discovery in 1967. Fig. 1 shows the general structural configuration of the reservoir, which consists of a large dome with faults oriented north-west/southeast. Faulting in the field, which partitions the reservoir into numerous small fault blocks, has caused some concern about implementation of the full-field development plan. Although lateral and vertical communication between upthrown and downthrown faulted blocks along major faults was suspected, it was never fully identified. Throughout the field's producing life, the extent of lateral and vertical communication within the reservoir, particularly between the oil zone and the underlying aquifer, has been questioned. This question became acritical operating consideration when a peripheral water injection plan was proposed in the late 1970's. Although an extensive reservoir study was undertaken to predict the field'sfuture producing performance under different waterflood operations, no waterinjection program was implemented. We decided that further detailed reservoir studies should be conducted to understand the communication and crossflow between fault blocks fully before a full-scale injection program could be designed. A preliminary 3D, coarse, full-field simulation study of the Thamama reservoir with an in-house simulator having a rectangular (orthogonal) grid system showed the significant effect of faulting on reservoir performance. Unfortunately, the crossflow and communication levels between the fault blocks could not be determined accurately because the model could not represent the faults properly owing to simulator limitations. Therefore, we decided to use a simulator capable of handling distorted (nonorthogonal) grids, or corner-point geometry, to fit the faults and reservoir boundaries better. Two approaches were considered for constructing the model grid. The first approach uses a rectangular (orthogonal) gridding system that represents faults at block boundaries. As a result, reservoir geology and geometry cannot be represented properly. However, finite-difference techniques can be used in a straight forward manner with good accuracy. The second approach uses a nonrectangular (nonorthogonal) gridding system that accommodates accurate description of the fault pattern by placing block boundaries along fault traces. However, the nonrectangular gridding systems could lead to serious calculation errors.1 We conducted full-field simulation studies using both orthogonal and nonorthogonal grid models. The results of the history match of both models with the same input data were compared to determine the magnitude of the calculation error and differences in model performance.