Geological and Geophysical Characterization of the ......Characterization of the Mississippian Midale reservoir at the Weyburn Field, using cores, logs, and seismic data from a 15.5

Saskatchewan Geological Survey 1 Summary of Investigations 2005, Volume 1

Geological and Geophysical Characterization of the Mississippian Midale Reservoir, Weyburn Field, Saskatchewan

Nicole M. Pendrigh 1

Pendrigh, N.M. (2005): Geological and geophysical characterization of the Mississippian Midale reservoir, Weyburn Field, Saskatchewan; in Summary of Investigations 2005, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2005-4.1, CD-ROM, Paper A-14, 16p.

Abstract Characterization of the Mississippian Midale reservoir at the Weyburn Field, using cores, logs, and seismic data from a 15.5 km2 area, reveals the geological complexity of an arid tidal-flat depositional environment that has undergone burial diagenesis. Seismic data of the reservoir fail to distinguish the two main reservoir zones, while a geological characterization has identified facies control on the sweep of the reservoir and the recovery efficiency. Core studies reveal several distinct facies throughout the reservoir, including original depositional facies as well as present-day units that have undergone diagenetic alteration throughout the life of the reservoir. Cross sections based on the cores indicate complex intervals of disconnected, high-porosity shoals surrounded by lower porosity intershoals. Isopach maps generated from log studies show fluid-impeding features such as thick argillaceous deposits, anhydrite accumulation, and diagenetically altered facies creating local barriers both vertically and laterally within the reservoir. Injection and production data draw attention to problematic areas of injectivity with corresponding production responses and can be correlated back to observed facies types and distributions. Poor injectivity and production rates are consistent with poor-quality reservoir rocks. The results and observations from cores, geophysical logs, and production data can be effectively combined and compared to the seismic data. Thus, seismic attributes can be related back to characterization and distribution, improving placement of wells and recovery rates within a thin reservoir.

Keywords: carbonate tidal flat, Marly, Midale reservoir, Mississippian, multi-component time-lapse, reservoir characterization, seismic attributes, Weyburn Field, Williston Basin, Vuggy.

1. Introduction An integrated geological and geophysical study using data from the Midale Pool of the Weyburn Field was conducted incorporating core, log, and seismic data from the Weyburn Field to identify carbonate facies and potential flow properties associated with these facies. Extensive alteration of the primary facies has occurred by secondary processes and diagenetic overprint is widespread. However, the recognition and prediction of facies are still the primary mechanisms to understand the geology of the Midale reservoir at Weyburn Field.

The first step in the investigation was a core study. Two main reservoir zones, the Marly and Vuggy, were discerned. Stacking patterns within the zones were studied to define primary facies patterns that may influence fluid flow (i.e., flow units). Open vug networks, open vertical and horizontal fractures, and other structures were observed in the cores including stylolites, cemented fractures, mineralized (in-filled) vugs, and argillaceous laminations but it was concluded that these features could not be linked to specific log signatures so efforts were concentrated on developing the linkage between logs and the facies signatures in the cores. Logs were cross-plotted and correlated with the core descriptions in an attempt to reveal relationships such as facies type and the effect of replacement anhydrite on the gamma-ray signature. A study area of 15.5 km2 (6 mi2) containing wells with cores and logs was chosen and these studies were integrated with seismic data. Together these data were used to develop a characterization of part of the Midale reservoir that is undergoing enhanced recovery.

1 Colorado School of Mines, 1500 Illinois Street, Golden, CO, U.S.A. 80403; E-mail: [email protected].


2. Geological Background Information

a) Depositional Environment The Weyburn Field is located in southeastern Saskatchewan (Figure 1) and covers an area of approximately 181 km2 (70 mi2) (Churcher and Edmunds, 1994). The Mississippian-age Midale Pool in the Weyburn Field is situated approximately 1402 to 1433 m (4600 to 4700 ft) below the surface. Below the base of the reservoir are the Frobisher-Alida Beds, and the top is bounded by two more shallowing-upward sequences (the Ratcliffe and Poplar Beds), capped by the Sub-Mesozoic Unconformity (Figure 1).

Discovered in 1954, the Midale Pool at Weyburn is a 1.3 billion barrel in situ reservoir (Yamamoto, 2004). Waterflood recovery began shortly after production started, with peak production being reached in 1966. A carbon dioxide miscible flood began in 2000, largely through a Simultaneous but Separate Water and Gas (SSWG) injection program.

The Midale Beds are interpreted as having formed in an arid tidal-flat environment formed on a low-relief carbonate shelf. The ramp contained a supratidal sabkha, a lagoon that formed on the inner shelf with restricted circulation and an outer shelf involving normal marine circulation. On the outer shelf, a relatively narrow bank margin formed which acted as an energy baffle for the shelf. High sediment-production rates occurred on the bank, and grain shoals formed. Typically the shoals were semi-continuous along the bank margin, with breaks caused by tidal currents. Over time, these environments changed in spatial position due to sea-level variations that controlled shallow-shelf deposition.

The Midale Beds are one of four peritidal shallowing-upward cyclic deposits (Churcher and Edmunds,1994). The Midale Beds consist of three main zones: a lower limestone unit referred to as the Vuggy; an upper dolostone unit, the Marly; above which is the Midale Evaporite. These three components formed an aggradational package with lithotypes typical of an arid tidal-flat depositional environment.

Sea-level affected deposition. During sea-level lowstand, the platform was exposed, and there was limited deposition. The exposed sediment was cemented in place, or dissolved away. Meteoric diagenesis occurred and caliches and evaporites formed. Transgressive deposition was generally thin, and was synchronous with platform flooding. The maximum volume of sediment deposition occurred during the highstand. As sea level fell, shoals,

Figure 1 - Location map and stratigraphic column of the Weyburn Field area (provided by PanCanadian/EnCana Corporation and adapted from Churcher and Edmonds, 1994). The Phase 1a CO2 flood area is outlined in pink. The control area is part of the Reservoir Characterization Project (RCP) study area. The Midale reservoir and basal and caprock evaporite units are highlighted in yellow.


which built up during highstands or stillstands, cut off circulation to the lagoon. Reflux dolomitization occurred, and platform interior evaporites formed.

b) Diagenesis Various stages of diagenesis are observed within the Weyburn Midale Pool (Churcher and Edmunds, 1994). The most obvious and extensive diagenetic process is dolomitization observed in the upper reservoir (Marly) unit. Near-surface contemporaneous dolomitization is a very basic reaction, and was probably an active process in the reservoir carbonates at Weyburn, where fluid flowing through the deposited limestone exported calcium and imported magnesium. The only fluid with sufficiently high magnesium concentrations was seawater, and near-surface processes such as storms, waves, tides, and currents provided a ‘pump’ to replace the pore volumes. Arid tidal settings provided a prolific environment for dolomitization. The key was the precipitation of sulphides, such as gypsum and anhydrite, which increased the Mg/Ca ratio. With intense evaporation leading to increased density and sinking, a pump was provided for hydrological circulation and a source for Mg-rich water.

In the study area, shoals presented a barrier to marine circulation creating a broad, high-salinity lagoon and reflux, a return flow of concentrated brine, occurred through the sediment of the lagoonal floor and across the barrier. Early dolomite formed from this process recrystallized, masking the original calcareous nature of the sediment.

Dolomitization can also occur in the deep-burial environment with high-pressure and high-temperature fluids, where retrograde solubility favours dolomite precipitation. However, very little new dolomite forms in the burial environment. It is likely that the primary limestone porosity is the greatest controlling factor for the development of dolomite porosity. Matrix dolomitization with organism dissolution can significantly increase the porosity, leading to increased storage capacity.

Anhydrite cementation is the dominant form of cementation in the Midale Pool at Weyburn. During burial, meteoric phreatic diagenesis occurred, and fluids were expelled creating syneresis and fluid-escape features. Displacive anhydrite in bird’s eye structures and in vugs greatly reduced porosity (Kent, 1984). Late diagenetic events included compaction and pressure solution with deep burial. Compaction and cementation led to stylolitization and associated development of insoluble impenetrable residue (stylolites can act as conduits or barriers to fluid flow).

Metasomatic anhydrite was the latest of all diagenetic changes in the reservoir (Kendall and Walters, 1978). Metasomatic anhydrite, largely consisting of brown porphyroblasts, replaced carbonate grains adjacent to fractures and stylolites. It preferentially replaced grainstones and other facies with high porosity and permeability and was spatially related to bedded anhydrite. Where present, metasomatic anhydrite commonly makes up 90% of the rock volume. It greatly reduces porosity and may cause reservoir compartmentalization. The porphyroblasts are concentrated within veins and along stylolites, and permeate outwards into the matrix, and were probably formed when brines penetrated along the veins and fractures. In the Midale Beds, metasomatic anhydrite forms in relation to oil migration and emplacement, indicating its development may have occurred as recently as the Late Cretaceous (Kendall and Walters, 1978). It is largely concentrated in the Vuggy zone, associated with the peloidal/oolitic grainstones, indicating that grainstone shoals are more of a contributing factor than bedded anhydrite. When metasomatic anhydrite is found in veins and fractures, the veins and fractures are often bounded by stylolites (Kendall and Walters, 1978).

3. Core Study For the purpose of the core study, the Reservoir Characterization Project (RCP) area can be subdivided into four quadrants (Figure 2). Cores were chosen for their distribution throughout the control area, with four wells in the west, three in the south, two in the east, and four in the north. Core depths were compared to log depths to determine if there was a shift in depth from the coring process.

The original schematic geological model depicts a lower Vuggy shoal of varying thickness overlain by upper Vuggy intershoal sediments followed by the Marly dolostone. During the core study, areas were observed within the Vuggy where tight lime-mud rock was found lower in the core than high-porosity packstones and grainstones; the shoal/intershoal complex is actually several stacking sequences where a thin shoal may overlie intershoal sediments. This indicates that, although the entire Midale interval may be a shallowing-upward or regressive sequence, the sea level rose and fell various times during the deposition of the entire 30 m thick reservoir beds. These sea-level cycles may be delineated as the study progresses and shoal/intershoal complexes are identified. For example, in any tidal-flat environment, the lagoonal facies is deposited simultaneously with shoals. However, in core from wells 16-13-6-14W2 and 2-24-6-14W2 from the Weyburn Midale reservoir, we see the shoal facies 7.6 to 15.2 m (25 to 50 ft) below the lagoon facies.


Figure 2 - Map of the RCP study area and Enhanced Oil Recovery (EOR) infrastructure highlighting the seismic survey area, and showing the areal distribution of studied cores (Yamamoto, 2004).

Deposition was mainly aggradational as sea level fluctuated. We observe in core that the shoal and intershoal sediments were deposited along the ramp, and that the lagoonal sediments were deposited on top of the shoals after sea level fell significantly. Sea level continued to fall, so the evaporitic sabkha deposits moved seaward and blanketed the lagoonal facies.

The Marly/Vuggy contact, based on gamma-ray and resistivity well logs, correlated closely with the first appearance of vugs in most of the cores. It is apparent that this contact represents an exposure surface with porosity and permeability values that change across it, in places inhibiting fluid from flowing from one zone to another.

At the scale of the study area, and for the purpose of monitoring fluid flow, the heterogeneity throughout the reservoir, evident from studying the core, is an important factor. Most wells show 3 to 4.6 m (10 to 15 ft) intervals of massive, oil-stained, bioturbated, microskeletal, dolowackestone/mudstone with varying degrees of fracturing, and argillaceous laminations in the lower Marly zone (Figure 3).

The facies in the Vuggy zone range from ostracod-peloid mudstone and wackestone (intershoal) to skeletal lime packstone and peloidal lime grainstone (shoal). Core studies show that small-scale stacking patterns from shoal to intershoal are recognizable, suggesting the presence of high-frequency shallowing-upward events. Also, stylolites, fractures, and anhydrite

Figure 3 - Marly zone: a laminated marly layer in dolo-wackestone in core from well 4/8-13-6-14W2.

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plugging are common. Often, stylolites are associated with fractures, especially those filled with anhydrite. Metasomatic anhydrite is also common throughout the Vuggy (Figure 4).

4. Core Analysis Using information obtained from studying the cores, facies were correlated between wells. The Marly and Vuggy zones were readily identified in both the geophysical logs and cores. Once the larger zones had been identified, smaller intervals were studied and correlated including the widespread flooding event that occurred in the lower Marly and the shoal/intershoal distribution within the Vuggy.

From studying the cores, the following nine facies types were differentiated: 1) supratidal evaporite (Upper Midale), 2) evaporitic outer lagoon (Three Fingers), 3) evaporitic inner lagoon (Upper Marly), 4) protected lagoon (Marly), 5) interbedded storm event (Marly), 6) intershoal mudstone (Vuggy), 7) intershoal/shoal slope wackestone (Vuggy), 8) shoal packstone (Vuggy), and 9) shoal grainstone (Vuggy). Figure 5 shows the location of these facies types in relation to the tidal-flat environment.

One of the nine facies types was assigned to each 0.3 m (1 ft) of core and the resulting logs were used to create cross sections. Based solely on these cross sections, various flooding events were identified in the Marly, and vertical stacking of shoals was identified within the Vuggy. Also from the core logs, wells that contained mud-dominated versus grain-dominated Vuggy intervals were discerned.

By using core interpretations to create structural depth slice maps every 1.5 m (5 ft) from 1399 to 1448 m (4590 to 4750 ft) below the KB elevation, a preliminary schematic diagram of facies relative to present-day structure was created.

Figure 4 - Vuggy features: metasomatic anhydrite within lime mudstone (left photo); jagged stylolite and an argillaceous lamination with a vertical burrow in lime mudstone (right photo). Both core photographs are from well 4/8-13-6-14W2.

2.5cm7cm


Figure 6 shows that the vertical and lateral distributions of shoal deposits versus intershoal deposits are complex. The shoals are generally not interconnected and are separated by intershoal facies with varying degrees of porosity and permeability. However, natural fractures may act as high-permeability conduits between the shoals. In the Marly unit, the top interbedded dolostone/anhydrite layer known as the Three Fingers (so called because of it’s characteristic gamma-ray log signature) and several argillaceous beds (including what is referred to as the Marly Flood Event [MFE]) possibly act as barriers to fluid flow. Four structural cross sections were created based on the core observations: north-south, east-west, and southwest-northeast.

5. Well Log Study Geophysical logs were compared to results from the core observations in order to enhance vertical resolution. For correlation, gamma-ray, neutron, sonic, SP, and resistivity logs were used. Of the various porosity logs within the log suite, the log that best fitted the gamma-ray, density, and core logs was the PhiE log. However, this log required careful calibration. Previous research (Herawati, 2002) has shown that typical Marly porosity is 16 to 38%, and

Figure 6 - Schematic structural cross section based on core observations. Note the vertical exaggeration.

Figure 5 - Facies distribution: numbers correspond to facies types based on core observations. Shoal model: Shoals (blue); Sabkha (purple); Intershoal break where tides and storms transport sediments in to and out of the protected lagoon (grey arrow). Typically shoals develop on up to tens of metres (tens to hundreds of feet) scale vertically, and kilometers (miles) laterally.

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Wackestone8. Shoal Packstone9. Shoal Grainstone

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Vuggy porosities range from 8 to 20%. By back-calculating the porosities from the original density log, it could be determined whether the PhiE porosities are reasonable and whether they fitted between these ranges.

Based on the calibration, a log value at a specific Vuggy limestone interval corresponds to 10% porosity. It is important to note that anhydrite within the Vuggy limestone will affect porosity values (in this case, 15%) which can be compared to higher values on the gamma-ray logs. The porosity log correlated to cores should, therefore, show where there is extensive anhydrite plugging. Figure 7 shows the gamma-ray, porosity, and core logs for the reservoir interval. The base of the Vuggy shows extensive pervasive anhydrite.

Figure 8 shows a cross plot of gamma ray, porosity, and facies type. The points are colour coded to facies types. The cross plot reveals relationships between the log values and facies types. For example, the anhydrite facies (red) range from gamma-ray values of 30 to 35 (API) with porosity values of 12 to 18%.

Based on the cross plot, porosity and gamma-ray values are summarized in Table 1.

6. Well-Log Analysis Isopachs created from log correlation can be used to help identify the areal distribution of units such as shoals within the reservoir zones. The cross sections can be compared to the original sections based on the core data.

A stratigraphic southwest-northeast cross section going through wells 4-13-6-14W2, 10-13-6-14W2, and 16-14-6-14W2 helps to discern features within the Marly (Figure 9).

The base of the Marly is mostly a massive, bioturbated, skeletal dolostone interpreted to have been deposited in a calm lagoonal environment. A major flooding event midway into the Marly is apparent in the logs and cores throughout the study area. The Marly Flood Event (MFE) is the only major marker observed in most logs and cores within the Marly. It is interpreted as representing a rapid sea-level rise, as a result of which argillaceous deposits blanketed most of the lagoonal sediment surface. This was followed by a sea-level fall, where normal lagoonal deposits formed again. The MFE ranges in thickness

Figure 7 - Gamma-ray, porosity, and core log for well 4/8-13-6-14W2. Tops were picked based on the gamma-ray and PhiE logs. The red rectangle highlights where anhydrite is present based on all three logs.

Figure 8 - A cross plot of gamma ray (y-axis), porosity (x-axis), and facies type (secondary y-axis).

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0.002 0.042 0.082 0.122 0.162 0.202 0.242 0.30

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Table 1 - Summary of gamma-ray and porosity values for some facies types.

from 0.3 to 1.8 m (1 to 6 ft). From southwest to northeast, the argillaceous flooding event deposits increase in thickness.

The lithology of the upper Marly zone varies from the reservoir-quality massive dolostone to an interlayered dolostone evaporite event, the Three Fingers, that is widely present in the northeast. Because of low porosities and permeabilities, the Three Fingers unit is unproductive. However, it is not present at all wells (e.g., the well at the left in the cross section in Figure 9).

A south-north structural cross section through wells 10-12-6-14W2, 1/7-13-6-14W2, and 16-13-6-14W2 clearly shows variations within the Vuggy zone (Figure 10).

Within the Vuggy zone, porosities are lower in the north, meaning shoal development was more productive in the south; mudstones and wackestones dominate in the north. Additionally, high-frequency shoaling-upward cycles (up

Figure 9 - Structural southwest-northeast cross section based on the porosity (PhiE) log. Top and base of Marly Flood Event (MFE) are indicated by the purple lines and the shallowing upwards sequences within the Vuggy are indicated by the blue lines. Note vertical exaggeration.

Facies Type Number Plotted Colors Gamma-ray Range Porosity Range (%) Marly dolostone 4 and 5 Green 18 to 35 6 to 29 Vuggy mudstone 6 Blue 10 to 30 3 to 13 Vuggy wackestone 7 Purple 8 to 20 8 to 13 Vuggy packstone/grainstone 8 and 9 Dark purple 11 to 12 6 to 13 Laminated and nodular anhydrite 10 Red 30 to 35 2 to 3 Stylolites 11 Grey 22 to 25 2 to 3 Argillaceous laminations 12 Brown 16 to 20 10 to 16

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Figure 10 - Structural south-north cross section based on porosity (PhiE) log. Note vertical exaggeration.

to six) are evident. Figure 10 shows an abrupt change in log signature at the Marly/Vuggy contact. This is interpreted to represent an exposure surface where sea level fell rapidly allowing for lagoonal deposits to accumulate directly on top of ramp deposits.

Isopach maps were made for various zones. From an isopach map of total Vuggy thickness (Figure 11), we see thickening to the south. From cores and logs, we see that high-frequency stacking patterns produced well developed shoals in the south part of the study area.

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Figure 11 - Isopach maps of a) total Vuggy thickness, ranging from 16 to 30 m and b) Marly Flooding Event (MFE) ranging from 0.5 to 2 m.

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An isopach of the Marly Flooding Event, or MFE (Figure 11) shows that the flood event is thickest in the northern area. This may be because the thick Vuggy deposition in the south prohibited substantial Marly deposition. There is also a thick deposit in the western region at a well where core evidence shows altered grainstone at the top of the Vuggy.

Vuggy grainstone shoals that were lithified from exposure remained thicker during Marly deposition than Vuggy intershoal deposits, which compacted more, allowing for thicker Marly deposits. The Vuggy topography created space for thin lower Marly deposition in the south and east, but thicker deposition to the north. It is also apparent that, in the west, thicker Marly dolostone deposits were surrounded by Vuggy thicks. Essentially, the topography of the lower zone controlled the deposition of the upper zone.

The presence of low-porosity MFE beds creates a barrier for vertical fluid flow. The fluid may still be able to flow laterally along the MFE beds, so where they are missing, vertical fluid flow is possible. Fluid flow above the MFE depends on the quality and type of rock above and below. At most wells, logs and cores reveal that, below the MFE beds, the Marly rocks are massive, highly bioturbated, and typically moderately to heavily oil stained.

To characterize the upper Marly zone, two isopach maps were generated: the Three Fingers ‘poor reservoir’ unit, and the massive dolostone ‘good reservoir’ unit (Figure 12). The second of these maps was created by subtracting the Three Fingers thickness from the upper Marly interval thickness. The Three Fingers isopach shows two anomalies: one in the north, where evaporite deposition apparently took place in an arid tidal-flat environment – the sabkha margin – and the second in the south-southwest. One theory is that this deposition occurred on the seaward side of a topographic high, leading to aerial exposure and evaporation. In contrast, the ‘good reservoir’ massive dolostone shows a thick accumulation to the west.

Where anhydrite is thickest, lateral flow is impeded. An examination of the MFE beds and the Three Fingers unit indicates that in the northern quadrant, where there is an argillaceous layer up to 1.5 m (5 ft) thick and an anhydrite layer 3.66 m (12 ft) thick, injected fluid has nowhere to go and any in situ fluid will be difficult to sweep.

At the Marly/Vuggy contact, a grainstone shoal was subaerially exposed when sea level fell. This can be seen by comparing a map of the top 1.5 m (5 ft) of the Vuggy (based on core observations) immediately below the contact (Figure 13, right) with an isopach map, based on logs, of the uppermost shoal sequence (Figure 13, left).

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Figure 12 - Isopach maps of the upper Marly (above the MFE). On the left, poor quality reservoir with thick anhydrite (Three Fingers) deposits. On the right, good quality reservoir dolostone.


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Figure 13 - On the left, an isopach map based on well logs of the uppermost grainstone shoal within the Vuggy. On the right, an facies map based on core observations of the top 1.5 m (5 ft) below the Marly/Vuggy contact surface (M, mudstone; W, wackestone; P, packstone; and G, grainstone). Both maps show a similar anomaly in the west (note there is some offset), interpreted as a grainstone shoal immediately below the contact.

These maps suggest that, at the end of Vuggy deposition but before Marly deposition, sea level dropped leaving a shoal exposed and susceptible to freshwater percolation in the west; where there was low-permeability mudstone at the top of the Vuggy, the sea level drop had little impact.

Upon examining the plotted anomalies evident on the isopach maps, it appears that ‘bulls-eyes’ are formed around the wells indicating a sampling bias. A correlation to seismic maps is needed for a more realistic characterization and distribution of the facies between wells.

7. Seismic Attributes Three multicomponent seismic surveys were acquired at Weyburn Field by the RCP. The first, or baseline survey, was acquired before CO2 flooding began in October 2000. Subsequent monitor surveys were acquired in October 2001 and October 2002 (Yamamoto, 2004). The reservoir is represented by half a wavelength or less in the seismic data, making the two reservoir zones below seismic resolution. An RMS amplitude time slice at a two-way travel-time of 1160 ms (Figure 14) shows that the reservoir gently dips to the south.

Figure 14 - Time slice at 1160 ms reveals structural dip and two possible faults (black lines) trending northwest-southeast and northeast-southwest generated in Landmark/Seisworks. The colour scale units are RMS amplitudes.


While present-day structure is apparent, the time slice reveals little about the stratigraphic controls or geological variations within the reservoir. The fingering feature in the east may imply a fault.

To understand which geophysical attributes are directly related to the reservoir geology, cross plots were generated. Attributes for cross plotting include p-wave impedance and p-wave amplitude. From studying previous geophysical maps, certain anomalies are identified, but they require a geological reference to be fully understood and explained.

a) Amplitude A result of the reservoir’s being represented by less than a half wavelength is that the Marly and Vuggy are averaged or smeared together by seismic attributes. To understand this effect and to relate the seismic data to logs and cores, the porosity values were averaged over the entire Midale reservoir interval on the PhiE log, and then cross-plotted versus the seismic p-wave amplitude. Two trends were revealed when porosity versus seismic amplitude were cross plotted (Figure 15). Trend 1 (lower amplitude, higher porosity) can be summarized as a thin-layered Marly, thin-stacked grainstone layers, and abundant anhydrite at the base of the Vuggy. Trend 2 (higher amplitude, lower porosity) is characterized by fairly continuous Marly dolostone, well defined MFE, packstone at the top of the Vuggy, and thick packstone layers.

The p-wave amplitude map (Figure 16) shows a low-amplitude anomaly in the west and a high-amplitude anomaly in the central and northern regions of both the Marly and Vuggy intervals. These anomalies correspond to porosity/facies variations observed in logs and cores. Because the Vuggy zone is typically twice as thick as the Marly zone, the seismic amplitude primarily represents Vuggy porosities. The high amplitude anomaly corresponds to observed lower porosities in the Vuggy, and fewer thinly stacked shoals. Consequently, it may not be apparent which reservoir zone is represented by the amplitude maps. However, we can determine that high amplitudes correspond to low porosities in the Vuggy based on the cross plot and a Vuggy average-porosity contour map (Figure 16, right).

The high-amplitude anomaly in the north-central region on Figure 16 corresponds to a mud-dominated Vuggy zone based on core and log observations, as seen in the cross sections in Figures 9 and 10. The low-amplitude anomaly in the west indicates a grain-dominated Vuggy. More importantly, low- to medium-amplitude values are present in the south, indicating that the high-frequency, high-porosity lower Vuggy shoals as seen in cross section in Figure 10 are taking CO2. Based on injection data, not only is the Vuggy taking the CO2, but the fluid is permeating outwards, indicating an absence of local barriers. Core and log data from this area reinforce this model.

Figure 15 - A cross plot of log porosity vs. seismic amplitude values. The red points are wells where the core was analyzed in this study; the green points represent other wells within the RCP study area.

Amplitude vs. Average Porosity

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Figure 16 - P-wave amplitude map of the reservoir interval, left. High amplitudes are red, and low amplitude values are blue. Colour scale is based on absolute amplitude values. Vuggy average-porosity contour map on the right.

b) Acoustic Impedance Rock properties that may affect acoustic impedance (AI) values are porosity, rock type, and fluid saturation. Cross plots of computed acoustic impedance versus PhiE porosity and facies show linear trends for both the Marly and the Vuggy zones (Figure 17). Again, it is difficult to separate the two zones within the seismic data.

Since the cross plots seem to correlate well to log and core properties, the next step is to look at the acoustic-impedance maps (Figure 18). The impedance maps were generated by inversion by Herawati (2002) and show the Marly and the Vuggy impedances for the baseline survey in 2000. This survey was acquired before CO2 injection. Because the wavelet shows impedance contrasts at the reservoir level, it is likely that the impedance values correlate to Marly density. The northern anomaly corresponds to a thick layer of anhydrite and the Marly Flood Event.

Although the impedance map looks different from the p-wave amplitude map, some comparisons can be made. For instance, in the west, the impedance maps show lower impedance and lower amplitude anomalies are present. In the north-central region, high impedance values occur and, although there is some offset, high-amplitude values are also present in the same area. Core and log evidence points to low-porosity Marly deposited over low-porosity Vuggy sediments.

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0.002 0.042 0.082 0.122 0.162 0.202 0.242 0.282 0.32155000

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0.002 0.022 0.042 0.062 0.082 0.102 0.122 0.142 0.162 0.182 0.2175000

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Figure 17 - Acoustic impedance cross plots for the Marly zone (left) and the Vuggy zone (right). The x-axis is PhiE porosity, and the y-axis is calculated acoustic impedance based on sonic and density logs.


Figure 18 - Acoustic-impedance maps based on the baseline survey. Note the high impedance values in the north-central region (red), and the low values in the west (Herawati, 2002).

8. Time-lapse, Injection, and Production Enhanced oil recovery from the Midale reservoir at the Weyburn Field is largely achieved through a Simultaneous but Separate Water and Gas (SSWG) injection program illustrated in Figure 19, whereby CO2 gas is injected into horizontal wells in the upper reservoir zone (Marly). However, the affects of fractures, anhydrite in-filling, stylolites, and argillaceous layers have not been considered in detail. It has generally been assumed that, while there may be some porosity and permeability differences between the two zones, there are no major fluid-flow barriers.

Figure 20 represents the collective amount of CO2 injected into the four horizontal wells within the RCP study area over a two-year period (October 2000 to August 2002). The greatest amount of CO2 has been injected into the southern well. In contrast, less than a quarter of that amount has been injected into the northern well.

Figure 19 - A schematic cross section of the SSWG injection program (courtesy of EnCana Corporation). The dashed line separates the upper and lower reservoir zones. Note that the schematic does not differentiate fluid movement from one zone to another.

MidaleAnhydrite

Oil & CO2CO2

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Hz CO2 Injector

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VerticalProducer

Vertical WaterInjector

Hz Producer

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It is now evident that these injection amounts are based on the localized geological differences. We know that in the southern quadrant there are high-porosity, well developed shoals with few anhydrite-filled vugs within the thick Vuggy zone, and little to no argillaceous Marly Flood Event beds, and massive oil-stained dolostone within the upper and lower parts of the Marly zone. The horizontal well in the northern quadrant was drilled into Marly zone with a thick argillaceous unit and abundant anhydrite within its upper part. The resulting low porosity and low permeability explain the low injection rate; the reservoir contains essentially no accessible pore-space into which

the fluid can be injected. Furthermore, lower porosity intershoal sediments characterize the Vuggy zone in the north. Fractures are present in the Vuggy zone, but metasomatic anhydrite and infilled vugs reduce the permeability. Clearly, the northern producing horizontal wells show poor response to the injection program, while the southern and eastern producing wells show good responses, as shown by the fact that CO2 breakthrough was observed in a vertical well adjacent to the southern pattern (Yamamoto, 2004).

Acoustic impedance difference maps of the Marly and Vuggy (Figure 21) show up to 10% changes in impedance at the injectors and producers with high CO2 injection rates and good responses. Little to no changes are observed near the northern injector. The most visible distinction is in the western injector, where no change is seen in the upper Marly, but there is a significant difference in the Vuggy (Figure 20). Within the Marly zone, the MFE beds are present, along with several smaller argillaceous units. However, the Vuggy zone contains both shoals and fractured intershoals, as well as open- and infilled vugs. Core studies suggest that the change in impedance seen here indicates that CO2 is not entering the Marly, but is finding a pathway, probably through vertical fractures, that allows CO2 to migrate downwards into the lower Vuggy and/or underlying Frobisher.

Figure 21 - Acoustic impedance difference maps from the first monitor survey and the baseline survey (Herawati, 2002). The largest difference between the two reservoir zones is apparent at the western injector, where little fluid is observed within the Marly, but radiates outwards from a vertical well within the Vuggy.

Figure 20 - Cumulative injection rates from October 2000 to August 2002 (courtesy of Sandy Graham, EnCana).


9. Discussion and Conclusions This study shows that it is difficult to differentiate facies solely on the basis of seismic data alone. An integrated investigation is necessary.

Core-based cross sections display a complex Vuggy interval of disconnected shoals surrounded by lower porosity intershoals. Although fractures may be present within the intershoals, they typically have lower permeabilities, which may not prevent flow but considerably reduce it.

Log-based isopach maps show both vertical and lateral features that impede flow. Such features include thick deposits of the MFE in the northern and western parts of the study area, thick anhydrite accumulations in the upper Marly in the north and southeast, and an anhydrite-altered (and plugged) shoal at the top of the Vuggy creating a local barrier at the Marly/Vuggy contact in the west.

Injection and production data highlight areas of high injectivity with a good production response where both zones contain facies types with rock properties consistent with good reservoir-quality rocks such as massive dolostones and porous shoals.

Distinguishing between the Marly and Vuggy from seismic amplitude or even impedance data is problematic. However, seismic anomalies are reflected by rock-property changes in both the Marly and Vuggy zones. For example, areas of high impedance indicate the presence of low-porosity Marly rocks, where fluid flow seems impeded both vertically and laterally – CO2 injection in such areas should be avoided since the fluid has nowhere to go, and virtually no communication exists between the injectors and their adjacent producers.

This study can now be extended beyond the limits of the RCP area, into the nine-pattern CO2 injection area to determine areas of high- and low-production rates based on the new geological information. Finally, based on the correlated core, log, and seismic data, a new 3-D geological model can be created to provide a visual representation of the complex facies variations and fluid-impeding features observed in this study.

10. References Churcher, P.L. and Edmunds, A.C. (1994): Reservoir characterization and geological study of the Weyburn Unit,

southeastern Saskatchewan: Report Number 1, Proposed Miscible Flood, Horizontal Well, and Waterflood Optimization Areas; internal report prepared for PanCanadian Petroleum Ltd., 28p.

Herawati, I. (2002): The use of time-lapse P-wave impedance inversion to monitor a CO2 flood at Weyburn field, Saskatchewan; unpubl. M.Sc. thesis, Colorado School of Mines, Golden, 153p.

Kendall, A.C. and Walters, K.L. (1978): The age of metasomatic anhydrite in Mississippian reservoir carbonates, southeastern Saskatchewan; Can. J. Earth Sci., v15, p424-430.

Kent, D.M. (1984): Carbonate and associated rocks of the Williston Basin: Their origin, diagenesis, and economic potential; S.E.P.M Short Course, Denver, 137p.

Yamamoto, H. (2004): Using time-lapse seismic measurements to improve flow modeling of CO2 injection in the Weyburn Field: A naturally fractured, layered reservoir; unpubl. Ph.D. thesis, Colorado School of Mines, Golden, 198p.

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Geological and Geophysical Characterization of the ......Characterization of the Mississippian Midale reservoir at the Weyburn Field, using cores, logs, and seismic data from a 15.5