Understanding Sequence Stratigraphy Architecture

Published Date: 02 Nov 2017

1 Geology Department, Faculty of Science, Suez Canal University, Ismailia, Egypt

2 Institute of Reservoir Characterization and ConocoPhillips School of Geology and Geophysics, University of Oklahoma, Norman, USA

Abstract

Understanding sequence stratigraphy architecture in the incised-valley is a crucial step to understanding the effect of relative sea level changes on reservoir characterization and architecture. This paper presents a sequence stratigraphic framework of the incised-valley strata within the late Messinian Abu Madi Formation based on seismic and borehole data. Analysis of sand-body distribution reveals that fluvial channel sands in the Abu Madi Formation in the Baltim Fields, offshore Nile Delta, Egypt, are not randomly distributed but are predictable in their spatial and stratigraphic position. Elucidation of the distribution of sandstones in the Abu Madi incised-valley within a sequence stratigraphic framework allows a better understanding of their reservoir quality evolution during burial.

Strata of the Abu Madi Formation are interpreted to comprise two sequences, which are the most complex stratigraphically; their deposits comprise a complex incised valley fill. The first sequence (SQ1) consists of a thick incised valley-fill of a late Lowstand System Tract (LST1)) overlain by a Transgressive System Tract (TST1) and Highstand System Tract (HST1). The second sequence (SQ2) contains channel-fill and is interpreted as a LST2 which has a thin sandstone channel. Above this, channel-fill sandstone and related strata with tidal influence delineates the base of TST2, which is overlain by a HST2. Gas reservoirs of the Abu Madi Formation (present-day depth ~ 3552 m), the Baltim Fields, Egypt, consist of fluvial late lowstand systems tract (LST) sandstones deposited in an incised valley. These sandstones have a wide range of porosity (15 to 28%) and permeability (1 to 5080mD), which reflect both depositional facies and diagenetic controls.

This study demonstrates the possibility of constraining and evaluating the impact of sequence stratigraphic distribution on reservoir quality evolution in incised-valley deposits, and thus has an important impact on hydrocarbon exploration in such settings.

Keywords: Sequence Stratigraphic, Reservoir Characterization and Architecture, Messinian, Abu Madi Incised-Valley, Baltim Fields

1. Introduction

Sequence stratigraphy provides a deterministic framework for understanding the relationships in time and space between rock layers, depositional environments and facies in terms of relative sea level changes, and a structured scheme to define reservoir architecture and reservoir quality (Catuneanu, 2006). Changes in the relative sea level allow constraining the quality as well as the spatial and temporal distribution of reservoir rocks within the context of sequence stratigraphy (Van Wagoner et al. 1990; Posamentier and James 1993; Posamentier and Allen 1999).

Incised valleys are considered important hydrocarbon reservoir targets because they may be filled with coarse-grained and, thus, porous and permeable fluvial deposits, that are commonly covered by sealing mud rocks (Posamentier and Allen 1999). Valley incision occurs as a consequence of rapid fall in relative sea level below the shelf edge, and consequent subaerial exposure of the continental shelf (Posamentier and Allen 1999). At an early, erosional stage, sediment bypasses the shelf to be deposited in deep-water environments. Termination of incision and overall onset of valley infilling with fluvial and estuarine sediments occurs when the relative sea level is stable at a lowstand position and/or begins to rise, i.e., during deposition of late lowstand and transgressive systems tracts (Van Wagoner et al. 1990).

Owing to their economic importance, incised valleys have been studied in great detail, with most works focused on the stratigraphy and architecture of incised-valley deposits (Dalrymple and Zaitlin 1994; Zaitlin et al. 1994; Willis 1997; Hou et al. 2003; Plint and Wadsworth 2003). Conversely, the reservoir-quality evolution pathways of incised-valley deposits have begun to be explored only recently (Dutton and Willis 1998; Ketzer et al. 2002). Dutton and Willis (1998) demonstrated that important diagenetic factors controlling the reservoir quality evolution of incised-valley sandstones include content of ductile grains and volume of quartz cement, which are more extensive in TST incised-valley fill estuarine sandstones, resulting in a greater permeability reduction during burial diagenesis than in LST fluvial incised-valley sandstones (Dutton and Willis 1998).

Additional case studies on sequence stratigraphy are thus needed to improve our ability to predict the reservoir quality and architecture of incised-valley sandstones. Therefore, the aim of this paper is to elucidate and discuss the sequence stratigraphy and related reservoir quality and architecture pathways of fluvial systems tracts sandstones that have been deposited in an incised-valley (Abu Madi Formation, late Messinian). Consequently, it is possible to obtain a better understanding of the internal complexity of the Abu Madi deposits based on seismic and borehole data.

The study area comprises the Baltim gas fields in the Nile Delta Basin, northern Egypt (Fig. 1).

2. Exploration History

Early stage (1966-1989). After the discovery of the gas-bearing sandstones of Abu Madi 1 well (1966), the late Messinian Abu Madi Formation represented one of the main targets in the present onshore area (Fig. 1). At the beginning, the discovery area was related to an elongated and faulted anticline, but the presence of different reservoir levels and the drilling of some dry wells suggested a mixed entrapment scenario associated with laterally discontinuous sandstone bodies. The Tertiary stratigraphy of the Nile Delta Basin was defined in these years (Sidi Salim, Qawasim, Abu Madi, and Kafr el Sheikh formations were described in exploratory wells of the area), and the first sedimentological interpretation referred the Abu Madi Formation to a continental/deltaic depositional environment (Fig. 2) . Several wells were drilled and the Abu Madi Field, now connected to the new discoveries of El Qar’a, started to reveal its present geometry and internal reservoir complexity (Dalla et al., 1997).

Second stage (1990-1993). The southward extension of this geological model confirmed the hydrocarbon potential of the late Messinian succession. More sophisticated seismic acquisition and processing were carried out in the East Delta Concession where, in 1990, the East Delta 1 well (Fig. 1) was successfully drilled, 25 km south of the Abu Madi Field. This and the other wells drilled in those years, either positive or dry, confirmed the strict relationships between gas accumulations and the Messinian drainage system, an ancestor of the present Nile River. An increasing amount of well data, the elaboration of a sequence stratigraphic model, and an effective technological improvement in seismic acquisition and processing, allowed to verify the regional geologic framework and to recognize more and more subtle traps (Dalla et al., 1997).

Third stage (1993-1996). The comprehensive multidisciplinary effort resulted in the projection of the Messinian targets into the offshore area and in the acquisition of a 3D seismic volume in the newly acquired concession of Baltim. After the discovery of gas, in 1993 (Dolson et al., 2001) in Baltim East 1 (Fig. 1), seven other wells were successfully drilled by the partnership IEOC/Amoco, delineating three fields in the area (Baltim North, East, and South Fields, Fig. 1). The Baltim exploratory phase represented not only the confirmation of a geologic stratigraphic model, but also a significant change in the geophysical interpretation (Dalla et al., 1997).

Also the regional picture became clearer and more reliable. Three different sequences, named UM1, UM2, and UM3, were defined and regionally correlated from East Delta to Baltim. All of them have shown a similar internal organization and evolution (Palmieri et al., 1996). The only major difference was related to the more prospective fluvial to fluvio-deltaic facies: sequence by sequence they were progressively deposited in a paleo-landward direction, i.e., backstepping during the late Messinian transgression (Fig. 3). All the fields were correlated at the regional scale, and the existing relationships among the different productive levels were highlighted, either by longitudinal continuity or gross facies distribution (Fig. 3).

3. Database and Methods

Our dataset from the Baltim fields consists of a set of 15 wells and 40 2D seismic line profiles provided by Petrobel, Inc., covering an area of 572 km2 (Fig. 1). The majority of the work was done by using normalized gamma-ray (GR) logs, deep resistivity (ILD), sonic (DT), neutron (CNL) and bulk density (RHOB). The work included identification and correlation of sequence stratigraphic surfaces, litho-saturation analysis, and construction of a subsurface cross-section of the Abu Madi Formation in the Baltim area.

A sequence stratigraphic framework was first established for the BE1 discovery well. This well contains the most comprehensive well database for the analyzed late Messinian stratigraphic targets, thus it was the reference well to build a well log sequence stratigraphic framework and then to extend it to the remaining area for the integrated interpretation. Once the key surfaces for stratigraphic correlation were identified, the well log signatures were extrapolated to the nearby wells through well log cross section correlations. Finally, the well log data and key surfaces were tied to the seismic data for seismic stratigraphic correlation and interpretation. Several loops between seismic and well log data were performed to assure agreement of interpretations. After the preliminary framework was established, seismic sequence interpretation was employed to more accurately delineate the reservoir intervals. This analysis formed the basis for identifying new opportunities in the study area and to plan a development strategy for Baltim fields.

Litho-saturation analysis and petrophysical core information were also integrated within this framework. Pressure tests from two wells were analyzed to test connectivity between the sequence stratigraphic intervals and verify the stratigraphic correlation.

4. Results

4.1. Identification and correlation of sequence stratigraphic surfaces

Based on borehole and seismic data, the strata of Abu Madi Formation are interpreted to comprise two sequences, which are the most complex stratigraphically; their deposits comprise a complex incised valley (Fig. 4). The first sequence (SQ1) consists of a thick incised valley-fill late Lowstand Systems Tract (LST1)) overlain by a Transgressive Systems Tract (TST1) and Highstand Systems Tract (HST1). The second sequence (SQ2) contains channel-fill and is interpreted as a LST2 which has a thin sandstone channel-fill. Above this, channel-fill sandstone and related tidally-influenced strata delineates the base of TST2 and overlain by a HST2. The main sequence stratigraphic surfaces that were identified and correlated (Fig.4 and 5) are the sequence boundary (SB1) at the base of the LST1 sandstone (i.e. the bottom of Abu Madi Formation), the sequence boundary (SB2) at the base of the LST2 sandstone, and the sequence boundary (SB3) at the base of the top of the HST2 (i.e. the top of Abu Madi Formation). The characteristics and recognition criteria of the following stratigraphic surfaces will be discussed later in this paper. The major valley directions in the study area are north-south (Fig. 5). Baltim South field is separated from Baltim East and North by a west-east directed normal fault. Valley incisions generally thin toward down-dip and thicken towards the up-dip directions. The thinning and thickening trends down-dip and up-dip can be observed in both subsurface well-logs and seismic profiles (Fig. 6) in the study area.

The quality of the available seismic allowed us to associate abrupt changes of reflection character with the lateral extension limits of the stratigraphic sequences. The use of complex trace analysis on 2D seismic data allowed a better characterization of the geometric relationships among the reflectors, and the delineation of an Abu Madi incised-valley became possible (Fig. 6). The Abu Madi Formation was interpreted as the sedimentary filling of this intra-Messinian erosional feature.

4.2. Litho-saturation analysis

Well logs allow the identification of key surfaces for sequence stratigraphic interpretation and for correlation of these surfaces between adjacent wells. Well log sequence stratigraphic analysis provides a first understanding of the relationships between the depositional environment and how it was affected by sea level changes. The gamma ray curve was used to identify log patterns and candidates for sequence boundaries, maximum flooding surfaces and other regional marker horizons (Fig. 4) were identified on seismic profiles.

The vertical distribution, in a form of litho-saturation plots, shows irregular vertical variations throughout the system tracts, in lithology, and water and gas contents (Figs. 7 and 8). Petrophysical parameters of the Abu Madi reservoirs, including shale volume, porosity and water saturation, vary from well to well in the Baltim fields. Figs. 7 and 8 show vertical variations for these parameters in wells BE1 and BN1, over the systems tracts of interest, The lithology of the Abu Madi reservoirs appears to be dominated by sandstone with more minor siltstone and shale, which are represented in LST intervals in each SQ1 and SQ2.

The pressure data obtained from RFTs are very useful as they enable judgments to be made about the position of the gas-water contact (GWC) in the Abu Madi reservoirs. Additionally, information is provided about compartmentalization, or whether the various fluids in a reservoir are separated physically by an impermeable barrier. The contacts between fluids (gas and water) were interpreted (Fig. 7 & 8) based on resistivity logs (gas/water) and formation pressure data (RFT). There are two different gas-water contacts in each well. In well BE1, GWC1 occurs at a depth of 3636 m while GWC2 occurs at a depth of 3700. In well BN1, GWC1 occurs at a depth of 3685 m while GWC2 occurs at a depth of 3778.3 m.

4.3. Porosity and permeability

Regardless of the quantity of wireline log data, numerous trials were done to understand the effect of porosity on permeability in the study area by constructing linear correlation cross plots of core porosity vs core permeability, and from this correlation, determining the permeability values for sequence 1 and sequence 2.

LST sandstones have a wide range of porosity (15 to 28%) and permeability (1 to 5080mD). In sequence 1 (SQ1), the data of the correlation between porosity and permeability from the cores (Fig. 9) reveals a systematic positive trend with LST1 sandstone having higher values of porosity and permeability than TST1 and HST1. Unlike sequence 1 (SQ2), there is no homogeneity in the LST2 sandstone (Fig. 10) and there may be three trends. Green data points of well BE4 exhibit a positive porosity-permeability trend in the middle of the thick LST2 channel facies. The second trend with orange colored points, from wells BN5 and BE3, exhibits a good porosity and permeability relationship, but the values are lower than the previous points. This trend is located near the periphery of the sand channel. The third trend with violet colored data points shows low values for both porosity and permeability in the BE5 well, which lies outside the channels.

5. Discussions

Reservoir characterization as a discipline grew out of the recognition that more oil and gas could be extracted from reservoirs if the geology of the reservoir was understood (Slatt, 2006). In fluvial reservoirs, when only sparse data is examined, oversimplification and questionable correlations of stratigraphic architecture are more likely. One of the means to constrain correlation involves using dimensional databases (e.g., Reynolds, 1999; Gibling, 2006 ; Pranter and Sommer, 2011 ). Data suggest that systematic changes in fluvial architecture and channel geometries may occur within sequences and systems tracts (Shanley et al., 1992; Shanley and McCabe, 1993, 1994; Van Wagoner, 1995). Reynolds (1999) showed clear changes in the width and thickness of fluvial channel sandstones in different systems tracts within sequences. In the Abu Madi of Baltim wells, sequence stratigraphic surfaces, markers, thickness trends, and lithology units were correlated using well logs and seismic data. Flooding surfaces within the Abu Madi shale and recognizable and widespread sequence boundaries at the top and bottom of the Abu Madi Formation were correlated and used to constrain the correlation of the sequence stratigraphic surfaces.

The overall incised-valley filling pattern of the Abu Madi Formation is consistent with an abrupt relative sea level fall followed by a stillstand, then a progressive rise, as described for other valley fill sequences (Posamentier and Allen, 1999; Shanley and McCabe, 1993; Catuneanu et al., 2009). The initial rapid relative sea level fall had an important tectonic-uplift component in the area (Sestini 1989), and caused valley incision and the formation of a regional unconformity at the base of the valley. The valley cut into the Messinian marine mudstones of the Qawasim Formation (Fig. 4).

Deposition of lowstand systems tract within the incised-valley occurred subsequent to stabilization of sea level at lowstand or an incipient rise, which favored the deposition of braided river and later meandering-river sediments. Sediment deposition occurred mainly along the axis of the valley, with some contribution from alluvial fans from the valley walls. The LST is a really restricted and laps out against the basin flanks. The internal architecture of the systems tract is characterized by sandy bed-load deposits organized as amalgamated, upward-fining and thickening channel-fill complexes. Eventually, high-frequency and high-amplitude fluctuations in base level, induced by relative sea-level changes, rapidly changed accommodation space within the valley. Transgressive systems tracts are characterized by a mixture of bed-load and suspension-load deposits arranged as upward-fining and upward-thinning bed sets. These strata have a wider areal extent than the lowstand deposits and are interpreted to reflect increased accommodation during periods of stratigraphic base-level rise. Periods of accommodation-space creation favored the development of meandering fluvial systems with widespread deposition of floodplain siltstones and mudstones (Wright and Marriott 1993) that now form reservoir seals at the field scale in the LST (Fig. 11). Recurrence of braided-river deposits suggests that accommodation space and/or sediment supply oscillated through time (Wright and Marriott 1993).

When the rate of relative sea-level rise and accommodation-space creation finally outstripped the rate of sediment supply, the system started to retrograde and the incised-valley was flooded with marine water and was transformed into an estuary. Consequently, the fluvial deposits were subjected to reworking by tidal processes. The tidal influence in incised-valley fluvial deposits has been used as a criterion to determine the beginning of overall shoreline transgression (Posamentier and Allen, 1999; Shanley and McCabe, 1993; Catuneanu, 2006) and thus marks the beginning of the TST in the Abu Madi incised-valley (Figs. 4 and 5). Deposition during early stages of the TST, within the valley limits, is dominated by siltstones and mudstones. These fine-grained deposits, which can reach up to 30m in thickness, constitute a stratigraphic marker and a reservoir seal. At this time, when the parasequences change from retrogradational to aggradational, the maximum flooding surface (MFS) is deposited (Emery and Myers, 1996; Catuneanu 2002). As transgression continued, tidal influence on the fluvial deposits increased until the point where the incised-valley was completely filled with sediments, marine conditions were established, and deposition of shelf mudstones took place. These mudstones (up to 150m thick) constitute the HST deposit and form an important stratigraphic marker in the basin (Fig. 11).

Characterizing reservoir heterogeneity is important for the understanding and optimization of the potential hydrocarbon accumulations in the Abu Madi Formation. Abu Madi reservoirs contain impermeable lithologic units and heterogeneous porosity/permeability distributions (Figs. 9 and 10), especially in the TST and HST, which are further affected by structural complexities that significantly can affect the fluid paths in the reservoir. Reservoir characterization is needed in effective petroleum production since reservoir heterogeneities are closely related to the pressure of trapped fluids. Floodplain deposits in each TST and HST represent a good seal separating LST gas reservoirs of SQ1 and SQ2 from two different gas-water contacts in each well in the Baltim fields (Figs. 7and 8). Baltim South field is separated from Baltim East and North by a west-east directed normal fault. This fault, is called the Baltim trend (Figs. 5 and 11). The Abu Madi incised-valley fill of this trend is complexly layered, with multiple stacked point-bar and estuarine deposits, which form structural and stratigraphic traps within the incised-valley fill (Dolson et al., 2001).

Conclusions

In the exploration and development activity, an effective geologic/geophysical interaction with a real methodological improvement, permits a continuous updating and improved understanding of the target reservoir. The mutual integration between a sequence stratigraphy interpretation of well data and seismic line profiles with semi-quantitative inferences, eventually resulted in a clearer and more precise picture of the Messinian Abu Madi Formation, and consequently, improved the success ratio of this subtle gas play in the Baltim fields. The information contained in this paper clearly demonstrates the importance of building a sequence stratigraphic framework for an improved understanding of the depositional model and physical distribution of reservoir properties in the Baltim gas fields of offshore Nile Delta, Egypt.

On the basis of extended bottom borehole logging and detailed well correlations, it was possible to recognize and describe fluvial to estuarine/shelfal facies, vertically arranged in an overall fining and thinning upward trend. The presence of sandstone bodies at different stratigraphic levels was related to smaller-scale regressive transgressive cycles within the regional transgressive trend of the late Messinian time. According to abrupt vertical facies changes and detailed seismic interpretation, different stratigraphic sequences were defined and traced over more than 572 km2. Due to the lack of enough comprehensive well data at the Messinian stratigraphic levels in the study area, the BE1 well provided the base for extrapolating geological and geophysical conditions away from the well site. The sequence stratigraphic framework developed for BE1 well was extended to 14 additional nearby wells taking into account the gamma ray pattern of the key surfaces and shale marker as datum. Once the key surfaces for stratigraphic correlation were identified, they were transferred to the seismic data to extrapolate them to nearby wells. Several loops between seismic and well log data were performed to assure agreement of interpretations.

Three sequence boundaries which bracket two depositional sequences SQ1 and SQ2, as well as two maximum flooding surfaces, were interpreted from the well sequence stratigraphy and extrapolated to nearby wells through well log and seismic correlation. Retrogradational parasequences sitting atop major sequence boundaries that are mappable both seismically and from well logs provides an important new play concept in the Baltim area.

Reservoir facies were deposited in incised-valley environments. In such environments, reservoir quality is driven by the amount of sand within the channel fill. As the sand content increases and the shale content decreases the porosity and permeability increase. LST fluvial channels represent the main reservoirs in Baltim area. Pressure data indicate that these reservoirs are mutually separated by TST and HST shale deposits. The geological model established here, as well as the nature and physical distribution of reservoir properties, was based on the sparse information available at the time this paper was initiated. Therefore, new knowledge should be included and the model regularly updated as new wells are drilled and new G&G data is acquired for Messinian objectives in this area.

The knowledge gained from this research is an important contribution to the petroleum geology of Baltim and the Nile Delta basin, which confirms the petroleum system for this Messinian play, defines a new play concept, establishes a sequence stratigraphic model for reservoir characterization and architecture if analogous projects are undertaken in the future, and defines the basis for further research in the study area.