INFLUENCE OF SOLID PORE FILLS ON RESERVOIR CHARACTERIZATION STUDIES CALIBRATED ON THE TRIASSIC VOLPRIEHAUSEN FORMATION SOUTHERN NORTH SEA BASIN

ABSTRACT

In the Buntsandstein gas fields of the Southern North Sea, the Volpriehausen reservoir is the main producing unit. This reservoir suffers often from the in-filling of the reservoir pore spaces by solid materials (salt plugs). Determining the reservoir quality of these fields based on seismic data alone has proved to be a major exploration challenge; the presence of salt-plugs in reservoir pore spaces seems to produce seismic responses that are similar at first sight to that of gas-filled reservoirs. Some wells drilled in the area have failed to yield the expected volumes of hydrocarbons because of this problem. Thus, in this study, seismic-based methodical approach was developed for reservoir characterization and modelling that could determine the difference (s) between solid-filled versus gas-filled reservoir intervals. To that effect, six methodical steps were adapted involving: Data (3D seismic, well and core) integration; Well data analyses involving well Log correlation, petrophysical evaluation and cross plot analyses; Seismic data analysis and interpretation; Development of rock- physical framework using petro-elastic modelling algorithm; Forward seismic modelling using 2D forward modelling simulator algorithm; and Amplitude Variation with Offset (AVO) analysis with AVO modelling algorithm. Two seismic surveys were merged in order to establish single global filters and scalars, and interpreted for subsurface structures and reservoir quality. Forward seismic modelling simulator algorithm was used for seismic simulation and verification calibrated in post- stack and pre-stack seismic domains. Amplitude Variation with Offset (AVO) modelling algorithm was used for AVO-based pore-fill characterization. Results of petrophysical evaluation of well data showed evidence of salt-plugging in the Volpriehausen reservoir, at intervals with anomalous log signatures between 3912 m and 3918 m. The petro-elastic modelling algorithm developed enabled fluid- and solid-substitutions based on numerical simulation. The substitution between gas, water and salt correctly predicted the elastic moduli and therefore the AVO behaviour of salt-plugged intervals. Also, by applying the new formula in performing fluid- and solid-substitutions on the seismic data,  subtle  differences  in  seismic  response  between  the  different  pore-fills  were  determined.    AVO modelling shows AVO response with clear distinctions between the different pore-fills, mostly expressed by the gradient behaviour. The significance of these findings is that the characteristic differences in elastic parameters and AVO attributes based on the methodical approach adapted in this study have proven  that  this  method  can  be  used  as  a  better  hydrocarbon reservoir  characterization  and prediction tool, as well as in correcting interpretation errors within the Buntsandstein gas fields, thereby preventing drilling into dry wells.

CHAPTER ONE INTRODUCTION AND MOTIVATION

1.1.     PROBLEM DEFINITION

In the Buntsandstein gas fields of the Southern North Sea, the Volpriehausen reservoir is the main producing unit. This reservoir suffers often from the in-filling of the reservoir pore spaces by salt plugs. Determining the reservoir quality of these fields based on seismic data alone has proven to be a major exploration challenge due to this phenomenon of some reservoir pore spaces being filled by tiny salt plugs. The presence of salt-plugs in reservoir pore spaces seem to produce seismic

responses that are similar at first sight to that of gas-filled reservoirs. Some wells drilled in the area failed to yield the expected volumes of hydrocarbons because of this problem. Prior to this study, seismic  (AVO)  signals  were  interpreted  based  on  conventional  substitution  (fluids  only)  as described by the Gassmann equation, leading to misinterpretation and resulting in drilling into dry (salt-plugged) wells.

A new approach in reservoir modelling that would clearly differentiate seismic behaviours of gas- bearing and salt-plugged reservoirs. Successful prediction of hydrocarbon volumes in place even in the presence of salt-plugging of reservoir pore spaces is then imperative.

This study was therefore set up in order to gain better insight into the subtle differences in the seismic response of water-, gas- or salt-filled Volpriehausen reservoirs within the lower Buntsandstein gas fields of the Southern North Sea Basin.

1.2.     RESEARCH OBJECTIVES

The objectives of the study were to:

i.    establish evidence of salt-plugging in the Volpriehausen reservoir.

ii.  develop  a  modelling  algorithm that  can  enable  solid  and  fluid  substitutions  based  on extended Gassmann’s theory.

iii. determine the impact of the solid in-fills in reservoir pore-spaces on Amplitude Versus

Offset (AVO) theory.

iv.  Apply the method developed in characterizing the Volpriehausen reservoir in the presence of salt plugs in the Buntsandstein gas fields. The results of the method should ultimately lead to better reservoir characterization and prediction of hydrocarbon reservoirs, rather than drilling into dry salt-plugged reservoirs.

1.3.     METHODOLOGY AND WORKFLOW

The research adopted a geophysical method, the applied seismology technique. Seismic, well and core data were integrated in order to validate the geological setting of the Volpriehausen formation, and to delineate the top-and-bottom of the reservoir. A number of industry-standard software were used for data interpretation, reservoir modelling and characterization and these include: RokDoc® (reservoir modelling and characterization), Geographix® (Data integration and geological model- building), Seisworks® (building the geological and petrophysical models), Petrel®  (Seismic data interpretation and 3D static modelling) and Matlab® (matrix laboratory for scientific programming and plotting). Some of the terminologies used in this study have been taken from the user guides of such software.

Figure 1.1 presents an integrated workflow adopted for this study. The workflow includes: (1)

integration of seismic, well and core data and in order to validate the geologic scenarios of the

formation, characterize the diagenetic features which control the heterogeneity of the reservoir and delineate  the  top-and-bottom  of  the  reservoir,  (2)  Analysis  of  well  data  for  the  qualitative assessment  of  logs  consistent  with  forward  modelling  and  depth-related  effects  such  as compaction, diagenesis and facies changes, which were incorporated into the rock-physical modelling algorithm (3) Development of Rock Physical Framework, constraining the elastic parameters of pores-fills, (4) Seismic simulation and verification on poststack and prestack domains using   2D   forward   modelling   (5)   AVO   attributes   modelling   for   AVO-based   pore-fill characterization.

Core

(section, logs)

Well

(composite logs)

Seismic

(Full 3D Prestack)

DATA SET

Core samples-based matrix-density extractions

Data integration (investigate specific reservoir of the Formation)

>Time surface

>Velocity model

>Time-depth conversion

>Depth surface

INTERPRETATION

Stratigraphic Correlation

Well Logs Analysis

Rock physics petro- elastic modelling Algorithm

Rock-Physical

Framework

Conceptual and

Solid-Fluid

Substitution

Programming using empirical

2D forward modelling simulator algorithm

Forward Modelling

Seismic simulation and calibration

Original wells

Artificial wells

AVO Modelling algorithm

AVO Modelling

AVO-based pore-fill characterization.

Final Model (reliable; based on which drilling should be into less dry wells

Figure 1.1. Flow chart of the study showing the workflow adopted.

1.4.     THE RESEARCH DATA SET

When constructing reservoir models, each piece of information has its own characteristic scale at which it provides information. No single source of information determines the

reservoir uniquely. In this study however, the data set used include Seismic (full 3D prestack volume), well (complete suite of logs including seismic velocities) and core (sections, logs and matrix density).

1.5      LOCATION OF THE STUDY AREA

The  Volpriehausen Formation  is  found  within  the  Northern part  of the  Southern North  Sea, Offshore, the Netherlands (Figure1.2). The study portion (the red square in Figure 1.2) of the Southern North Sea  survey lies approximately within longitudes 4.00-6.000E and latitudes 53.5-

54.50N and is well-accessible from the Zuid Holland region of the Netherlands.

Figure 1.2. Inset map (green rectangle within the black square at the top left corner) of the study area showing the study as located offshore Netherlands within the Southern North Sea. Red rectangle indicates the licensed block of the gas fields.

1.6.    REGIONAL GEOLOGY OF THE STUDY AREA

1.6.1.  Stratigraphy

The general stratigraphic successions are summarized in Table 1 (Geluk 2007). The Triassic is subdivided into two groups with the following lithostratigraphic descriptions:

1.  The Lower Germanic Trias Group (latest Permian– Olenekian), comprising mainly fine- grained clastic deposits with sandstone and oolitic intercalations, with predominant content of sandstones at the southern basin margin;

2.  The Upper Germanic Trias Group (Olenekian–Norian) comprising an alternation of fine- grained clastics, carbonates and evaporites with subordinate sandstones.

The boundary between these groups is formed by the Hardegsen or Base Solling Unconformity, which forms a regionally well-correlatable event (Rӧhling and Geluk, 1999; Geluk, 2007). The Volpriehausen Formation, which is the primary target in this case study, is located in the Triassic Lower Germanic group.

The top of the Upper Germanic Trias Group, i.e. the base of the Rhaetian Sleen Formation, forms an excellent marker on both seismic data and well logs. The Sleen Formation, representing the youngest Triassic, belongs to the Altena Group (Wong, 2003).

*Table 1: Stratigraphic subdivision of the Triassic in the Netherlands and adjacent countries.

* From Geluk (2007) and Kozur, 1999. Ages after ISC (2003); Sequences after Gianolla & Jacquin (1998); transgressive sequences in black, regressive sequences in grey. EK I: main Early Kimmerian Unconformity, base Norian; EK II: Early Kimmerian II Unconformity, base Rhaetian; H: Hardegsen Unconformity. * The Middle Muschelkalk is an informal unit and comprises the Muschelkalk Evaporite and Middle Muschelkalk Marl.

1.6.2.  The Volpriehausen Formation

The Volpriehausen Formation is divided into two Members:

1.  The Lower Volpriehausen Sandstone (clean sandstone) and

2.  The Upper Volpriehausen Clay-Siltstone (several claystone intercalations).

There  is  no  clear  interface  between  the  Lower  Volpriehausen  Sandstone  and  the  Upper

Volpriehausen Clay-Siltstone, but more of a gradual transition.

The Volpriehausen Formation displays  its  largest thickness, over 200m,  in the  Dutch Central Graben and the Broad Fourteens Basin. It reaches 100 m in the Ems Low and 150 m in the Roer Valley Graben. The Volpriehausen Unconformity at the base of the formation locally cuts up to several tens of meters into the Lower Buntsandstein Formation. According to Geluk (2007), the thickness of the Volpriehausen reservoir in the study area ranges from a minimum of 4m (water well) to a maximum of 41m (gas well).

1.6.3.  Structuration

Structurally, two extensional tectonic phases took place during the Triassic, related to the disintegration of Pangea (Ziegler, 1998; Kockel, 1995):

– the Hardegsen phase which culminated during the Olenekian; it affected the thickness and sand dispersal patterns of the Main  Buntsandstein and comprises up  to  four short-lived rift  pulses (Rӧhling and Geluk, 1999);

– the Early Kimmerian phase during Anisian to Norian times, which affected the thickness and salt distribution of the Rot, Muschelkalk and Keuper formations and which comprises up to five pulses (Beutler, 1998). It resulted in two unconformities; the most important lies at the base of the Red Keuper Claystone (Early Kimmerian I), the second at the base of the Sleen Formation (Early Kimmerian II) ( Geluk, 2005).

Analysis of the subsidence pattern shows that differential subsidence was spasmodic and shifted northwards with time, from the Roer Valley Graben and West Netherlands Basin to the Ems Low and the Dutch Central Graben (Rӧhling and Geluk, 1999). This was accompanied by strong uplift of the Mid North Sea and Ringkobing-Fyn highs.

The fault pattern of this phase suggests an east–west extension, with minor strike-slip movements along a number of fault zones. The onset of the Early Kimmerian phase was in Anisian times, when differential movement started in the Dutch Central Graben. The Netherlands Swell was cut along a series of ESE–WNW trending  faults,  and  partly collapsed. The  Early  Kimmerian movements continued intermittently during much of the Triassic, as is illustrated by the differential subsidence of the Dutch Central Graben and the movements along the Mid Netherlands Fault Zone. The strongest movements occurred during the Carnian. During these movements the main rift was situated in the Gluckstadt Graben (Kockel, 1995). In the Netherlands, the Dutch Central Graben displayed the strongest subsidence. Basement faulting triggered widespread mobilization of Zechstein salt. Thick successions of the Muschelkalk and Keuper formations occur in the rim synclines of salt diapirs. Ensuing strong uplift and subsequent deep erosion occurred in the southern Netherlands, where a system of NE dipping fault blocks separated by WNW-trending faults was formed. The oldest deposits subcrop on the south-western parts of these blocks, the youngest on the northeastern parts (Figures 1.3a and 1.3b). Local intrusion of Zechstein salts occurred along the faults (NITG 2000, 2002). In the Dutch Central Graben, these movements triggered further widespread mobilization of the Zechstein salt and the collapse of salt pillows formed during the Early Triassic.

Apart from N–S trending faults, a system of WNW–ESE trending faults was active. These faults, for instance the Mid Netherlands Fault Zone and the North Dogger Fault Zone, are characterized by

(b)

(a) A

Figure 1.3. Structural map of the Buntsandstein group.

(a) Subcrop map of the Early Kimmerian Unconformity after Geluk (1999). Abbreviations, including those used in other figures: BFB: Broad Fourteens Basin; CBH: Cleaverbank High; CNB: Central Netherlands Basin; CNG: Central North Sea Graben; DFZ: Dowsing Fault Zone; DCG: Dutch Central Graben; EL: Ems Low; HG: Horn Graben; LBM: London-Brabant Massif; MNFZ: Mid Netherlands Fault Zone; MNSH: Mid North Sea High; NDFZ: North Dogger Fault Zone; RFH: Ringkøbing-Fyn High; RM: Rhenish Massif; RVG: Roer Valley Graben; WNB: West Netherlands Basin. (b): Section based on well data, showing the structure of the Triassic in the West Netherlands Basin. The base of the Sleen Formation serves as the reference level for this section. After Geluk (1999, 2005).

locally preserved Upper Triassic sediments in their hanging-wall blocks. Other faults (Gronau Fault Zone) mainly show uplift. They are interpreted here as transcurrent faults. The Triassic displays a marked difference in tectonic styles in the Netherlands (Figure. 1.4).

1.6.4. Petroleum system of the Volpriehausen Formation

The Triassic sandstones are widely distributed in the Netherlands and form, after the Rotliegend, the second most important gas reservoirs in the Netherlands (Jan de Jager et al, 2007).

In the southern part of the West Netherlands Basin, where no or very little inversion occurred, gas generation continues to present day. Although the Triassic play in the basin is largely a gas play (Figure. 1.5), in several accumulations (Papekop, Pernis West, Spijkenisse Oost, Waalwijk Zuid and Botlek) an oil leg was found underneath the gas, and the Ottoland structure probably contains oil only.

The oil has been correlated to the source rocks of the Lower Jurassic Posidonia Shale (Figure 1.6), with probably some contribution from the Lower Jurassic Aalburg and uppermost Triassic Sleen formations. This oil migrated into the older reservoirs from down-thrown blocks across faults.

Figure.  1.4. Section from the  Broad Fourteens Basin  illustrating the  seismic  character of the Triassic. The Lower Germanic Trias Group is a rather transparent unit, with several continuous low-amplitude reflectors in its upper part. The Upper Germanic Trias Group displays an alternation of low and high-amplitude reflectors. The collapse graben in this group developed during the Late Kimmerian extensional tectonic phases. In this structure, the Muschelkalk Salt forms the

detachment horizon. RO: Upper Rotliegend Group; Z3C: Z3 Carbonate. After M. C Geluk, 2007.

Figure. 1.5. Gas and oil fields in the Netherlands. Total reserves are dominated by the giant Groningen gas field in the north-east onshore. Study area is indicated by L and M blocks, predominantly gas fields.

Figure 1.6. Hydrocarbon systems in the Dutch subsurface.

Arrows show from which source rocks the main reservoirs have been charged with gas and/or oil. The Upper Permian Zechstein salt, present in much of the subsurface, provides a regional seal between a Paleozoic gas and a Mesozoic oil and gas system.

Offshore, the Volpriehausen and Detfurth sandstones are gas reservoirs encountered in the northern

Vlieland Basin, Terschelling Basin, Schill Grund High and southern Dutch Central Graben area.

In the northern part of the Western Netherlands basin, for example in block L6, similar reservoirs, that are of high quality in the Southern part, appear to be salt-plugged. The large thickness of the reservoir in part  of block L9  suggests a unique depositional setting. Extensive core coverage indicates that the reservoir is made up mainly of eolian sandstones, with locally made sediments deposited as a result of glaciomarine sedimentation.

Well and seismic data show that the reservoir interval is wedge-shaped, thickening into listric normal fault that detaches onto the top of the Zechstein salt, adjacent to a salt wall.

Salt plugging of the reservoirs is a serious risk to the Triassic plays in the northern offshore (Purvis

& Okkerman, 1996), hence the essence of this investigation. Salt-plugged reservoirs have been encountered in particular near salt walls and along fault planes, and are often characterized by a phase reversal of the seismic response at the top reservoir (Jan. de Jager and Geluk, 2007).

₦5,000.00 – DOWNLOAD FULL PROJECT DOWNLOAD FULL PROJECT Added to cart

---

---

---

---

---

Related Project Topics :