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3D geometric modeling has received renewed attention recently, in the context of visual scene understanding. The reservoir geometry of the Baltim fields is described by significant elements, such as thickness, depth maps, and fault planes, resulting from an interpretation based on seismic and well data. Uncertainties affect these elements throughout the entire interpretation process. They have some bearing on the geometric shape and subsequently on the gross reservoir volume (GRV) of the fields. This uncertainty on GRV also impacts volumes of hydrocarbons in place, reserves, and production profiles. Thus, the assessment of geometrical uncertainties is an essential first step in a field study for evaluation, development, and optimization purposes. Seismic data are best integrated with well and reservoir information. A 3D geometric model of the Late Messinian Abu Madi reservoirs in the time and depth domain is used to investigate the influence of the reservoir geometry on the gas entrapment. Important conceptual conclusions about the reservoir system behavior are obtained using this model. The results show that the reservoir shape influences the seismic response of the incised Abu Madi Paleovalley, making it necessary to account for 3D effects in order to obtain accurate results.

The Nile Delta Basin started to show its hydrocarbon potential in the early 1960s. Since then, generations of geologists and geophysicists have applied different concepts and methodologies to explore this area, keeping pace with the latest available technologies. Regional gravity surveys were followed by extended 2D seismic surveys up to almost a routine 3D acquisition in the 1990s [

Baltim area lies to the north of the Nile Delta between latitudes 31°37′25′′ and 31°56′19′′N and longitudes 31°1′12′′ and 31°26′7′′E, about 25 km off the Egyptian Coast. It covers an area of about 500 km^{2}, with a length of 25 km and a width of 18.75 km (Figure

Location map of seismic profiles and wells in the Baltim area (offshore Nile Delta, Egypt).

Baltim East was discovered in 1993 and the production started in April 2000. In the past couple of years some key workovers and a new slanted well (BE10) investigating the northern area of the field, in addition to the good field performance, highlighted the possibility of additional potential in the area and the inadequacy of the available 3D model to correctly simulate the producing behavior. Baltim North was discovered in 1995. The production started only in November 2005 with the tie-in of well BN1. Recently acquired data confirm a complex dynamic relation between the Baltim East and North fields. A new 3D seismic reprocessing has been performed in 2005 merging all the 3D data acquired on the area and producing four angle stack volumes [

In order to optimize the development plan in terms of number and placement of wells a detailed reservoir model capturing the complex internal geometry of the reservoir is required. Therefore, the aim of this work is to define the general geological setting of Abu Madi Formation, where gas and condensate accumulation have been trapped, and to construct 3D geometric model of Abu Madi sandstone reservoirs to help in determining the next locations for the future development of Baltim fields.

The Upper Miocene (Messinian) Abu Madi Formation consists mainly of sandstone intercalated with siltstone and shale interbeds. The Abu Madi Formation is a fluviomarine environment [

Lithostratigraphic column of the Nile Delta in Baltim area, Egypt (modified after [

In Baltim area levels III upper unit, III A, II, and I are shale-out. Baltim fields in the offshore Nile Delta are gas-condensate accumulations located in the northern portion of the Abu Madi Paleovalley area [

Schematic cross section illustrating the sequence stratigraphic framework of the Abu Madi Formation in Baltim fields, offshore Nile Delta, Egypt [

The general structural setting of the Delta area has been determined using both geophysical methods and well data. The main feature is the Nile Delta Hinge Zone [

Hinge zone structural feature [

3D geometric model of the Abu Madi reservoirs “level III main” and “level III lower” have been done by using Petrel program (Schlumberger’s Reservoir Modeling Software). The available data for the current study (Figure

One of the first steps in interpreting a seismic dataset is to establish the relationship between seismic reflections and stratigraphy [

Synthetic seismograms are artificial reflection records made from velocity logs by conversion of the velocity log in depth to a reflectivity function in time and by convolution of this function with a presumed appropriate wavelet or source pulse [

Well BE1, Depth-OWT relationship with linear depth scales. The impedance log, reflection coefficient, and synthetic seismogram generated using the sonic and density logs are included. Part of seismic line 2609 is plotted together with the synthetic seismogram at well BE1.

Structural interpretation is the most fundamental interpretation activity and includes making maps of horizons and 3D structural model. By correlating specific horizons on a seismic line, it can subsequently generate time data which, after conversion to depth, help generate structural maps (maps which show the geologic structure of a feature) and isochron or isopach maps (maps which show time or thickness of particular intervals, resp.) [

Interpreted arbitrary seismic line consists of T6685, L1967, and T6101, from south to north, showing the main four-horizon area ((1) bottom Abu Madi, (2) top level III lower, (3) top level II main, and (4) top Abu Madi) and the main two faults in the Baltim area.

Uninterpreted (a) and interpreted (b) seismic line number (L 2660) passing through Baltim East field from west to east direction, where (1) bottom Abu Madi, (2) top level III lower, (3) top level II main, and (4) top Abu Madi are picked.

Uninterpreted (a) and interpreted (b) seismic line number (L 2609) passing through Baltim East field from west to east direction, where (1) bottom Abu Madi, (2) top level III lower, (3) top level II main, and (4) top Abu Madi are picked.

Uninterpreted (a) and interpreted (b) seismic line number (L 1889) passing through Baltim North field from west to east direction, where (1) bottom Abu Madi, (2) top level III lower, (3) top level II main, and (4) top Abu Madi are picked.

The interpreted horizons in the seismic sections from base to top are as follows (Figures

Bottom Abu Madi: the interpretation follows a zero crossing value along a strongly angular unconformity at the base of the Abu Madi Fm. While the acoustic contrast strongly changes along this stratigraphic surface, the erosional geometry at its base and onlapping horizons above allows following it (although at times with uncertainty) at a regional scale.

Top level III lower and top level III main: the interpreted horizon is a seismic peak, locally continuous, whose amplitude is related to decrease in seismic velocity. The reflection’s strength changes significantly and is stronger where gas bearing sands determine a strong impedance reduction.

Top Abu Madi: the interpreted horizon follows a zero crossing value between a strong and continuous through-peak couplet representing a decrease in seismic velocity.

Depth conversion of a time interpretation is computationally simple and can be quickly repeated whenever new information becomes available. The physical quantity that relates time to depth is velocity. The velocity required for converting time to depth is the P-wave velocity in the vertical direction. It can be measured directly in a well, or extracted indirectly from surface seismic measurements, or deduced from a combination of seismic and well measurements [

The complete interpretation is automatically converted using Petrel software. The workflow of converting data between domains within Petrel is split into two processes:

make velocity model which defines how the velocity varies in space;

depth conversion which uses the velocity model to move data between domains.

The picked time values and the fault segments locations are posted on the base map of the study area in order to construct structure time maps for the studied horizons (top Abu Madi, top level III main, top level III lower, and bottom Abu Madi). Then, the velocity model is used to convert the reflection time to depths, in order to construct the structure depth maps.

Top Abu Madi has two-way time (TWT) varying between 2871 and 3349 ms, while depth values vary between 3372 and 3651 m (Figure

Time and depth structure maps of top Abu Madi Formation.

Time and depth structure maps of top level III main horizon.

Time and depth structure map of top level III lower horizon.

Time and depth structure map of bottom Abu Madi Formation.

The area was dissected by two main faults (Figure

Two isochore thickness maps were constructed for the two pay zones “level III lower” and “level III main.” Uncertainties affect these elements throughout the entire interpretation process. They have some bearing on the geometric shape and subsequently on the gross reservoir volume (GRV) of the Baltim fields. The increase of the gross reservoir volume (GRV) leads subsequently to the increase of the net pay thickness, volumes of hydrocarbons in place, reserves, and production profiles. For level III main the thickness varies between 0 and 190 m (Figure

Thickness map of level III main.

Thickness map of level III lower.

A simplified fluvial sequence stratigraphic model of the Late Messinian Abu Madi Formation is shown in Figure

Stratigraphic architecture of a fluvial depositional sequence influenced by base-level fluctuations for the Abu Madi incised valley channel system. Each system tract contains a fining upward succession caused by the continuous coastal aggradation and subsequent shallowing of the fluvial graded profile. List of abbreviations: lowstand system tract (LST), transgressive system tract (TST), highstand system tract (HST), maximum flooding surface (MFS), and sequence boundary (SB).

As apparent from Figure

Reservoir modeling is playing an increasingly important role in developing and producing hydrocarbon reserves. Various technologies used to understand a prospective reservoir provide information at many different scales. Core plugs are a few inches in size. Well logs can detect properties within a few feet around the well. Seismic imaging covers a huge volume, but its typical resolution is limited to a few meters vertically and tens of meters horizontally [

One of the key challenges in reservoir modeling is accurate representation of reservoir geometry, including the structural framework (i.e., horizons/major depositional surfaces that are nearly horizontal, and fault surfaces which can have arbitrary spatial size and orientation), and detailed stratigraphic layers (Figure

3D geometric model of Abu Madi reservoirs showing the promising pathway (red dashed line) of the future development wells, which coincides with the up-to-date locations of drilling wells in the Baltim area.

In typical structural modeling workflows, the first task is to build a fault network as a set of surfaces and contacts between these surfaces. This step is itself decomposed into surface fitting, which creates one fault surface from each fault interpretation, and editing in which faults can be extended, filtered, and connected one to another based on proximity and modeler’s interpretation. Then, horizons are built from seismic picks conformably to the fault network [

The final result is a 3D geometric model of Abu Madi reservoirs based on well and seismic data (Figure

In conclusion, we have seen that the role of geometric modeling is becoming more important for exploring reservoir structures. 3D geometric modeling provided a useful means towards understanding the structure of Abu Madi reservoirs. Baltim fields (South, East, and North) comprise two separate gas pools named “level III lower” and “level III main” within the upper Messinian Abu Madi Formation. The trap is a structural-stratigraphic type with pinch-out against incised Abu Madi Paleovalley boundaries and is fault-bounded in the northern and southern part. Identification of stratigraphic architecture not only helps understand the geological history, but also has implications for hydrocarbon exploration as the confinement of flow in an incised valley has great implications for channel amalgamation and produces favorable reservoirs with potential two-way closure. The Abu Madi reservoirs shape influences the seismic response of the incised Abu Madi Paleovalley, making it necessary to account for 3D effects in order to obtain accurate results. The accuracy of the estimated thickness of each Abu Madi reservoir is a critical element in assessment of reserves, volumes of hydrocarbons in place, and production profiles. The promising locations of the future development wells based on the integration of seismic and well data coincide with the up-to-date locations of drilling wells in the Baltim area and locate at the center of the Abu Madi Paleovalley. 3D geometric model of Abu Madi reservoir in Baltim area should be kept in mind during future field development decisions.

The authors declare that there is no conflict of interests regarding the publication of this paper.

The authors wish to express their gratitude to Egyptian General Petroleum Corporation (EGPC) and Belayim Petroleum Company (PETROBEL) for providing the seismic lines, well logs, and other relevant data. Ministry of Higher Education & Scientific Research and Ministry of Petroleum in Egypt are also acknowledged for promoting advancement in research and establishing a possible future linkage between the industry and university. They thank Schlumberger for furnishing the Petrel software for the seismic interpretation.