Sedimentology and Sequence stratigraphy in part of Block 8, Blue Nile Basin, Sudan

Objective and work program:

The main aim of this study was to construct a sequence stratigraphy framework of the Blue Nile Basin, to better understand the organization of the petroleum system and to delineate potential play concepts. To undertake this study, 5 wells were chosen to provide the optimal stratigraphic and areal coverage of the study area.

For the effect of performing the study, a series of tasks were then proposed by TOKAMAK as follows:

- Well log sequence stratigraphy interpretation has to be performed to define and described the parasequence and high order sequences depending on the log motifs, since there is no core data available.

- Perform a 1-D sequence stratigraphy analysis from well logs on the three wells Dinder-1, West Dinder-1 & Hossan-1 in addition to Jauhara-1 and Frasha-1, which located out of the study area.

- Correlate sequence sets between wells by extrapolating the sedimentary facies in terms of log motifs, since there is no core data coverage in the study area.

- The correlation between the wells conducted to assess the major controls on the depositional infill.

- Integration of the stratigraphic model with the 2D seismic data in order to finally integrate all the geological and geophysical data across the sequence stratigraphic panel.

Data collecting, loading and evaluation:

During the data collecting phase, WNPOC provided most of the available geophysical and geological data of Block 8 needed to undertake the study. The well data of five wells namely; Dinder-1, West Dinder-1, Hossan-1, Jauhara-1 and Farasha-1 were successfully loaded to the workstation for quality evaluations. Some discrepancies and missed data were observed in the log suits which affected the interpretation workflow summarized in table (1) below.

The data quality control included:- revising all well log curves and coordinates;- Creating a new Geoframe geological database;- Creating synthetics for all the available wells in the study area to

tie wells to seismic data.

The geological data provided by WINPOC during the data collecting phase was the following:

- An Excel list of 5 exploration wells, listed above, including the wells coordinates TD, Markers and RTE.

- Well logs in ASCII format for Dinder-1, West Dinder-1, Farasha-1, Hosan-1 and Jauhara-1.

- FMI image for the lower part of the Farash-1 well.- Petrophysical reports for most of the wells. - Final well reports.

Data Quality control:After data loading, a quality control for the data was undertaken. The QC of the geological data consisted in several major steps:

- All the available wells were checked for their log curves and coordinates some missed sections of the logs were observed as shown below.

- A systematic revision of the formation tops was undertaken, and it was observed that there is some mismatch between the formation tops extracted from the mud logs with those provided in a list; finally we follow the list provided by WNPOC.

- The FMI data which covered the Blue Nile formation in Farasha-1 well, which is located out of the study area, has a poor to bad resolution. Reprocessing of the image is highly recommended for the interpretation purpose.

Formation Dinder-1 Hossan-1 Jauhara-1 Farasha-1 West Dider-1Damazin 165 50 204 163Dinder I 366 259 508 410Dinder II 1868 998 1147 808 1932Dinder III 2616 1829 1872 1583Blue Nile 3456 2555 2680 2111TD 4242 2911 2978 3050 2030

Table (2) Summarize the provided Formation tops.

- The washout phenomenon dominated all the wells under study and affected the hall condition in different intervals, as a result most of the interpretation of the facies and correlation job been highly affected across these intervals.

Sequence stratigraphy:

As per the scope of work of the present study, the following chapter describes the proposed major tasks in a sequential manner.

Well log sequence stratigraphy;

A total of 5 wells were loaded to the workstation 3 of which were located in the study area. These well have a full or near full set of wire line logs covering the study interval. The wells represented in a key W-E section elaborated in the basin, which is geographically positioned in a regular pattern in order to establish the sequence framework. The list of the wells was located in table (2).

Definition of Genetic sequences for the studded wells;

For each of the three wells under study, a composite well log was created displaying the most representative electric logs together with a lithological analysis (based on Gamma Ray, Resistivity and Neutron-Density logs) summarized in table (3) below:

Table (3) electric logs used for facies interpretation and genetic units.

The lack of calibration data (Conventional core & FMI) for the formations under study make it difficult to propose a realistic and more accurate sequence stratigraphic scheme.

FMI interpretation:

No in-depth interpretation of the available FMI survey of Farasha-1 well has been performed for the scope of the resent study. Such an interpretation for a reliable sedimentological investigation requires a good quality image as well as a core to calibrate both facies and fracture features. Due to the lack of a core covering a complete FMI run, caution must be taken when interpreting such material. Moreover, additional information regarding paleocurrent data will not be of a value since the well is located far from the study area.

Sedimentary facies Associations from Well-log signatures:

Facies association and trends can be observed more clearly from log motifs. Basically the facies have been distinguished as belonging to four major facies associations that can be identified in logs:

1- A fluvial sandstone facies, corresponding to thin single-story channels, ranging in thickness from 2m to less than 10m thick; or thick multistory channels, 10m to 50m thick, with a marked fining-upwards trend.

2- A deltaic sandstone and alluvial to palustrine heterolithic facies, 10 to 50m thick, presenting mostly coarsening-up trends.

3- An alluvial overbank to palustrine mudstone facies, a few meters to 100m thick, with occasional paleosoils and bioturbations.

4- A lacustrine mudstone facies, encompassing lacustrine to lacustrine pro-delta environments.

The facies and facies associations are listed in table (3) below

Table (3) Summarize the facies association in from log motifs.

Sequence Stratigraphy Methodology:

The sequence stratigraphic concept applied to the Blue Nile Basin combines the base-level approach of Cross & Lessenger (1998) and/or Homewood et al. (2000) and structural models for extensional basins of Gawthorpe et al. (1997).Lithology from mudlogs and wireline logs used in the interpretation of sedimentary facies, which was combined with biostratigraphic data and provided the means for stratigraphic and sedimentological interpretations.

The stratigraphic base-level is an abstract (non-physical), continuous surface that rises and falls with respect to the earth's surface. Sediment accumulation occurs only when the base-level and the surface of the solid earth accommodation space is available. If the base-level is below the surface of the earth, sediments will be eroded and transported down slope to the next location figure (), where accommodation space is available.

In this sense, within sedimentary basins and specially within continental basins, the up-and –down movements of the base-level produce the sedimentary record. When base-level rises (base-level rise hemicycle), it's intersections with the basinward-tilted surface of the earth move up-gradient (uphill); thus, it creates more accommodation space, in both marine and continental environments (Cross & Homewood, 1997). In this accommodation space, sediment will be deposited if available if the base-level falls (base-level fall hemicycle) accommodation space decreases and sediments are eroded (when the base-level falls below the earth's surface).

Generally the base-level describes the relationship between processes that create and remove accommodation space, and processes that deliver or remove sediments from this accommodation space. Thus, for practical reasons, the movements of the base-level are explained by the interaction between the variation of accommodation space (A- space available for sedimentation in a certain time interval) and sediment supply (S - volume of sediments available in the same time interval). It is the ratio between accommodation space and sediment supply (A/S ratio) that controls the build-up of sediment sequences (Cross & Lessenger, 1998).

The creation of accommodation space is a function of the interplay between subsidence of the basin floor (tectonics, isostatic response to water and sediment load, compaction), lake and groundwater level fluctuations in continental settings.

During a base-level rise hemicycle, the creation of accommodation space exceeds the sediment supply (A/S increases). As a consequence, the depositional environments move up the depositional gradient (distal environments move into more proximal positions), forming an associated vertical rock succession in which facies becomes more proximal towards the top.

During a base-level fall hemicycle, the creation of accommodation space cannot keep pace with the sediment supply. As a consequence, the accommodation space will be filled and sediments bypassing and even erosion occurs, while the depositional profile is shifted basinward (down the depositional gradient). The associated vertical rock succession has more proximal facies towards the top and is characterized by surfaces of unconformity and/or in extreme cases no deposition is recorded due to intense by-passing.

The transition from base-level rise to base-level fall hemicycle (rise to fall turnaround) is characterized by the maximum A/S ratio. The rise-to-fall turnaround corresponds to a maximum flooding surface in marginal marine settings (in terms of the sequence stratigraphic nomenclature).

The minimum of the A/S ratio is recorded at the change from a base-level fall to a base-level rise hemicycle (called fall-to-rise turnaround) and displays an incomplete depositional record. If it is a major erosion surface, It could be regarded as a sequence boundary (in terms of the sequence stratigraphic nomenclature).

The correlation of base-level cycles within the Blue Nile Basin was only possible after its structural configuration was understood from seismic interpretation. Lateral variations in accommodation space and sediment supply were, to a large extend, controlled by the subsidence pattern of its individual fault blocks, as is also evident in other extensional basins. The complex basin geometry of the Blue Nile and particularly lateral subsidence variation in depositional strike and dip directions has been taken into account during the interpretation work.

Sequence Cycles Hierarchy:

Base-level cycles group strata that record the rise and fall of the base level. This implies the variation of depositional conditions in time and their shift on the depositional gradient. The cycles are bounded by rise-to-fall turnarounds.Different ranks of stratigraphic cycles are identified in the Blue Nile Basin and are represented on the three selected wells.

- C-III genetic sequence;- C-II genetic sequence sets;- C-I long-term cycles.

The criteria for establishing the cycle hierarchy include facies changes of depositional environments, change of depositional environments in stratigraphic section and the areal extent of cycle recognition.

The C-I, or long term cycles, are first defined in seismic, which can be identified on the seismic sections and are regionally correlative at the basin scale. They correspond to Major Tectono-sedimentary cycles (MTC).

The C-II, or genetic sequence sets, can only be correlated locally at the scale of faulted blocks and are allocyclic in origin. These cycles form the base of the sequence framework propagated at the scale of the studded area.

C-III, or genetic sequences, represented the finest sequence cycles identified in logs and can also be identified in cored material. They are autocycilic in origin and have only been defined in the individual selected wells. They are not correlative by nature.

Conclusions & Results of the Well log Sequence Stratigraphy of Blue Nile Basin:

The log signatures were used in order to:- Identify major lithologies (sandstone, shales) which are described

below by formation.- Perform correlations.- Link the lithologies to major depositional facies and environments.

Dinder I Formation:

The review of the log signatures in the Dinder and Dinder I intervals shows that these formations are mostly composed of fine to medium-grained sandstone, interbedded with claystones. This Lithology indicates a stacking of a single story fluvial channels facies association (e.g. meandering channels), corresponds to 2m to rarely 10 m thick, medium to fine-grained sandstone units, encased within mudstones and/or heterolithic succession. A clear fining-upward GR & density responses are characteristic. Sandstones are passing generally upward into overbank-type mudstones or hetroliths (O/P).

Dinder II Formation:

This formation is mostly composed of sandstone in the upper portion and shales in the lower section. The lower shales represents lacustrine facies associations, corresponds to thick monotonous successions of essentially high radioactive mudstones (high Gamma values) with a serrated GR log response. Sedimentary trends correspond to fining- and/or coarsening-upward trends. The intermediate and upper intervals dominated with intercalation of sandstone and mudstone which reveals delta-front sandstones to heteroliths facies association corresponds to a succession of thin fine-grained sandstone beds intercalated with mudstones interbeds. Sedimentary trends exhibit characteristic coarsening-upward succession.

Dinder III Formation:

The intermediate and upper section sections of this formation seem to be dominated with multi-story fluvial channels facies associations corresponds to 10 – 30m thick massive medium to coarse-grained sandstones, with low mudstone content. The upper most part of this formation exhibit a fining-upward trend and a transition to overbank fines

(O/P) deposits. Whereas, the lower interval dominated with lacustrine mudstones with high radioactive values and serrated GR log response. Dinder III reservoir deposited during the time of maximum fall of base-level, the deltaic elements are finishing their progradation, the fluvial channels reach their maximum erosion into underlying sediments (subareial unconformity was created) and the fall-to-rise turnaround is reached.

Blue Nile Formation:

This formation is mostly shaly and contains very few intervals of salts and/or thin deltaic sandstones. The log response indicates dominant lacustrine mudstones facies associations of essentially highly radioactive mudstones and serrated GR response. Sedimentary trends correspond to fining and/or coarsening upward C-III cycles due to subordinate occurrence of centimeter-thick fine- grained sandstones/siltstones beds intercalated/scattered throughout thick muddy unit. The general trend was a coarsening upward C-II cycle. The Blue Nile Source rock facies deposited during the maximum of accommodation space creation recorded within the basin (A/S ratio is high). At that time open/shallow lacustrine facies (L) may be developed in the most distal (basinward) areas and extensive flood plain (O/P) developed in inferred in the more proximal position accompanied by occurrence of few high sinuosity channels.

Formation tops correlation panel

Recommendations:

In future work it is recommended to:- Take a complete core covering a large stratigraphic interval, which

would allow the identification of the major facies, facies associations and source rock intervals.

- Perform a mineralogical study on complete core interval that would serve to identify the trace minerals that have important impact on the log signatures and allow a better calibration of the log analysis results.

- Run complete log suites in exploration and appraisal wells in order to supply a maximum amount of data on the formation Lithology and mineral content. Spectral GR logs (CGR, SGR, THOR, and URAN) and photo-electric factor logs (PEF) are associated to be additional to neutron, density, sonic and resistivity logs.

- Acquiring 3D seismic data will allow to integrate the seismic attributes and the seismic inverted data which proved to be very useful to detect the sand bodies and their connectivity, and will lead to a high accuracy depositional model and stratigraphic framework for further field development plans.