Narhari Srinivasa Rao

Understanding Fractures Through Seismic Data: North Kuwait Case Study

Summary: Understanding fracture corridors is the primary driver for successful development of fractured carbonate reservoirs. This assumes further significance if the carbonate reservoir is characterized by very low porosity and permeability; producibility of the reservoir is purely dependent on the presence of natural fractures. Distribution and type of natural fractures is a function of palaeo & present day stress, structural elements, regional tectonics and diagenetic history. Direct detection of fractures is below the resolution of conventional seismic data. However, through a combination of seismic derived attributes integrated with well data, it is possible to better understand the distribution of fracture swarms. Kuwait Oil Company (KOC) is currently engaged in an early phase of development of a tight fractured carbonate North Kuwait Jurassic gas play. Considering the limited well control, field development is heavily reliant on seismic data for fracture characterization. This paper presents our current understanding of the relationship between fractures observed in the well data and structures, faults and lineaments interpreted on seismic data. In addition to conventional seismic analysis a suite of seismic attributes including Dip, Coherence, Edge and 3D Volume curvature were used for mapping structures, faults and minor lineaments. Well-wise and field-wise analysis of relationships between seismic derived attribute-pattern and fracture orientation was established. The understanding between these two different sets of data has helped in locating potential zones of sweet spots for placing successful delineation and development wells. These seismic attribute volumes were also used as soft constraint for building the Discrete Fracture Network (DFN) model for populating the fracture network in the reservoir model. The data presented in this paper are from the Raudhatain, Sabriyah and North West Raudhatain (NWRA) fields for the Najmah-Sarjelu part of the Jurassic section.

Use of High Resolution Sequence Stratigraphy in Building 3D Reservoir Geological Model - A Case Study from Kuwait

A number of structures are producing hydrocarbons in commercial quantity from the carbonate reservoirs of Middle Marrat Formation in the Northern part of the State of Kuwait. Seismic resolution of these deep carbonate reservoirs of Toarcian age is strongly affected by multiples from overlying Gotnia-Hith salt-anhydrite layers impacting true understanding of the internal depositional architecture from seismic data making it difficult to make realistic geological models. An extensive coring campaign adopted over a period of time for improving the understanding of depositional settings and diagenetic processes in these reservoirs and subsequent studies helped in developing a correlation based on sequence stratigraphic principles. In the Middle Marrat Formation seven cycles with sequence boundaries and maximum flooding surfaces were identified. The best reservoir facies are located in High Stand Tract, corresponding to the progradation of the carbonate platform in the form of clinoforms sloping towards the basin. 3D geological model was built with a framework base on this correlation capturing the higher order sequences with the prograding and aggrading carbonate shelf. Sedimentary facies were identified from electrologs with complete calibration from core description and ECS calibrated ELAN processing results. Gross depositional environment maps were prepared to guide the distribution of primary sedimentary facies within the model for different depositional environments like Slope/Basin, Outer Shelf, Inner Shelf, Shoreface, Barrier/ Shoal, Backbarrier/ Backshore, Lagoon, Tidal Flat, and Sabkha using templates developed for different sequence boundaries. These carbonates are characterized by seepage reflux dolomitization process and burrows giving rise to enhanced porosity and higher productive layers. This diagenetic imprint was modeled on the primary facies in shoreface and inner shelf areas. This facies model was used to distribute other reservoir properties like porosity, permeability and water saturation using different techniques. The developed model was further calibrated with isotope geochemistry studies and was also validated from comparison with exposed and well studied geological analogue examples. This model showed better predictive capability for wells drilled subsequently. In this carbonate reservoir, a sequence stratigraphic framework holds the key for distributing the reservoir facies within a 3D geological model in a realistic manner giving better predictability of reservoir properties and facies thereby reducing the field development uncertainty.

Exploration Strategy For Unconventional Reservoirs In the Lower Cretaceous Section of Kuwait: Case Study

Kuwait Oil Company (KOC) the State oil enterprise of Kuwait holds exclusive rights to Exploration and Production in the State of Kuwait. As part of the strategy to add new reserves to its portfolio, KOC has embarked upon an aggressive exploration campaign of the hitherto less explored stratigraphic levels. The Lower Cretaceous stratigraphic section, a set of fracture associated unconventional reservoirs, is one such play currently under active exploration.

Field Development and Well Planning in Tight Carbonate Reservoir Using Fracture Characterization and In-Situ Stress Mapping From Core Reorientation Studies: Kuwait Case Study

Fracture characterization is vital for efficient field development of naturally fractured Carbonate reservoirs. Successful development of fracture reservoir in the study area was possible due to early recognition of fracture play right from the exploration phase and through careful execution of relevant data acquisition campaign in the initial stages of field development. Comprehensive and integrated studies have been carried out over the past few years to arrive at an understanding of the conceptual model in deciphering structural evolution of North Kuwait Jurassic. Extensive core and image log data was acquired in the initial stages of field appraisal, which helped in comprehensive forward planning in design of deviated and horizontal wells. The data analysis steps included accurate and reliable reorientation of the cores. These data were calibrated with the image logs, along with available seismic attributes, which resulted in better understanding of structural evolution and sweet spotting of horizontal wells. This reorientation of the core data also helped in establishing a number of quantitative fractures attributes such as frequency, spacing, dip-azimuth and aperture along with mapping of in-situ stress directions. The detailed integration of these data also helped in accurately mapping the local and regional present day stress and its variations spatially across the fields. Stress direction across the field was helpful for deciding the azimuth of wells during well planning along with selection of completion strategy for current set of horizontal drilling Campaign. Drilling and testing results have been encouraging through enhanced reservoir performance in these tight carbonate reservoirs, based on these integrated studies.

Classification and Environmental Distribution of Microbial Lithofacies: A Key Reservoir Description Tool

Most of the fossil and sedimentary structures that have been referred to as algal, cryptalgal, or cyanobacterial can now be grouped under the term microbial and are understood to have formed through the activities of this diverse group of prokaryotic organisms. Such photosynthetic biota, by removing CO2 from marine waters, induce the precipitation of carbonate minerals. Microbial lithofacies are extremely common in the mid-Mesozoid Toarcian Series, occurring in many different forms and along nearly all of the depositional profile, down to the base of the photic zone. They may also be transported farther basinward, into slope and basinal environments. Several authors have suggested classifications of microbial fossil forms. Virtually all of these forms can be recognized within the Toarcian of the Arabian Plate. Long cored intervals in more than 40 wells provide complete coverage of the calciclastic shelf depositional profile from tidal flats to at least outer shelf. Microbial forms are particularly abundant in most wells and provide the basis for a morphological classification based on this model. The subdivisions of this morphological classification are described and illustrated in this paper. Their distribution along the depositional profile is summarized, based on lithofacies, interpreted depositional environment, and ichnofacies data.

Complex Carbonate Evaporite Reservoir Description Using Isotope Geo-chemistry and Ichno-facies to Fine-tune a High Resolution Sequence Stratigraphic Framework Model of Marrat Reservoirs

Field development in complex carbonate evaporite reservoirs has been a challenge for geoscientists by using simple sedimentology based geological models. A high resolution sequence stratigraphic framework has been designed from a conceptual 3D depositional model to a deterministic predictable model. In this process framework boundaries have been fine-tuned with carbon and oxygen isotope signals and defining ichno-facies associations to map lateral continuity of member formations within Marrat sequence. As dolomites and porous grain stones are the key reservoir quality rock types for Marrat Group of formations, position of precursor lithofacies in sequence stratigraphic frame work is found to be crucial. Dolomite samples are analyzed for d13C and d18O. Upper and Lower Marrat members are characterized by thinly laminated, micro crystalline exposure dolomites and high in d18O signal which are associated to low stand packages, whereas Middle Marrat dolomites are relatively low in d18O and high in d13C are associated to high stand precursor facies. Moreover, typical ichnofacies association of porous dolomites defines vertical and lateral connectivity of flow zones in Middle Marrat reservoirs. The reliability of this model has been established through recent tested intervals, with PLT and MDT data integration demonstrates predictability of flow zone connectivity.

Use of High Resolution Sedimentological Tools for Core Lithofacies identification and their Grouping of Association

Detailed sedimentological core description manifests the heterogeneity in lithofacies distribution and reservoir quality characterization in a complex carbonate-evaporite succession. To produce such detailed core drafts, in addition to routine sedimentogical tools and protocols, polished core slab high-resolution scans have been used to identify surfaces, hardgrounds, ichnotraces and microbial assemblages. A standard lithofacies catalogue has been prepared to guide intrawell facies interpretation and interwell interpolation to build a composite sequence stratigraphic model. 26 core-based lithofacies have been identified and grouped in to 10 depositional facies associations along a depositional profile varying from ramp to calciclastic shelf-setting in an intra-shelf basin with limited syndepositional tectonic relicts. Approximately 15000 ft of core slabs from 75 wells have been processed and interpreted to build profile relays showing changing position along the depositional profile and interpreted vertical stacking pattern. This work flow proves to be a best practice in reservoir modeling where framework- building and distribution of precursor facies between sequence-bounding surfaces is the key for successful field development and reservoir management.

Plan View Seismic Interpretation - Applications In Kuwait

Deviation from exploration for conventional, Albian age, clastic plays towards carbonate plays of a stratigraphic nature has prompted the use of plan view - horizontal seismic displays in the interpretation process as an additional means of data visualization enhancement.