The Egret Member

In this note I want to focus on the Egret Member (Fig. 1). This unit belongs to the Rankin Formation and is interesting from economical point of view.

The Egret Member contains the Upper Jurassic (Kimmeridgian) rocks.

It is easily to detect this member both downholes and geochemical logs. It is because the Egret Member rocks content higher amount of organic carbon (TOC total organic carbon) in comparison  with another deposits. Average TOC content is up to 12 % (Fowler and McAlpine, 1993).

Fig. 1 Litostratigraphic column and geochemical log of Jeanne d'Arc Basin (from Fowler and McAlpine, 1993).

The thickness of the Egret Member is laterally changing and range from 55 m (Rankin M-36) to 226 m (Fortune G-57)(Fig. 2).

According to McAlpine (1990) who subdivided sedimentary fill in the Jeanne d’Arc Basin into six main depositional sequences, the Egret Member belongs to epeiric basin (in my last note it is described as the First Thermal Sag).  

Fig. 2 The Jeanne d'Arc Basin. Grey dots mark wells which reached or penetrated the Egret Member (from Huang et al., 1996).

To the southern boundary the Egret Member is predominantly represented by laminated brown marls intercalated with calcareous mudstones (Rankian-M36) and  to the northwest gradually become dominant slightly calcareous shale, siltstone and minor sandstone (Trave E-87).

Examinations based on cutting samples coming from Trave E-87 and Fortune G-57 let to determine 4 types of lithology, which represent all the Egret Member:

1. Dark brown laminated shale (the richest in organic matter)
2. Light brown to grey shale
3. Marlstone/limestone and claystone
4. Sandstone (subordinate)

Lithologies alternate with each other rhythmically. TOC also varies. Diagrams of TOC show organic rich layers alternated with organic poor layers. Lithology and its thickness is vertically changing. Examination of samples under the microscope showed that the most of organic matter is amorphous (Fig. 3). 

Fig. 3 Percentenge of litologies from cutting samples examnations. A- carbonate, B- shale/claystone, C- siltstone, D- sandstone. TOC diagram. Measurements of Archer K-19 well. The broken lines indicate boundaries od the Egret Member (after Huang et al., 1996).

Depositional Environment of  the Egret Member

The Egret Member rock were deposited in low energy marine environment. It is suggested by fine-grained and laminated deposits and high organic content.

According to isopach maps Egret Member deposits are the thickest in the central part and on the Outer Ridge (eastern part of the basin)(Fig. 4). Between this two zones occurs zone with thin deposits. This suggests that it was sill there. The sill probably acted like a barrier and circulation was curbed. The highest value of isoliths of carbonates occurs in the southern part which suggests a closeness of carbonate shelf or bank. Occurrence on the south of oolitic and skeletal packstones below and above member also suggest carbonate shelf. The Rankian Formation, excluding the Egret Member was deposited in normal marine conditions (based on examination of microfossils). The Egret Member was formed in shallow waters with depth about 25-50 m. It was probably anoxic basin, which is suggested by occurrence of ostracods, which can live in extreme environment, and lack of foraminifera.

The content of terrestrial organic matter is low and it indicates that delivery from continent was minor. Both factor: restricted circulation and high plankctonic productivity (especially by dinoflagellates) led to creating suboxic and anoxic conditions in the bottom waters. High amounts of organic matter were accumulated. As I mentioned before the amorphous organic matter is dominant it is because after accumulation during early diagenesis was reworked by anaerobic bacteria. 

Fig. 4 a) isopach map of the Egret Member B) isolith map of carbonate of the Egret Member (after Fowler and McAlpine, 1993).

Sedimentary cycles in the Egret Member

The Egret Member has a cyclic nature. Examination on this aspect was carried out by Huang et al. (1996).  They use variations of TOC and permeability which was calculated from well logs. Estimated permeabilities  obtained from different vertical distances was put on variograms. Results confirmed cyclicity of sedimentation (Fig. 5). 

Fig. 5 Semi-variogram of permeability.  Hibernia K-18 (from Huang et al., 1996).

With the use a mirror display of the gamma ray Huang et el. (1996) visually identified cycles. Minima of gamma ray defined cycle boundaries. It show the distinctive cyclic variations in the Egret Member which are probably related to alternating layers of different lithologies such as clay, silt and TOC content.

In thick source rock intervals (Archer K-19) can be determine three different distinctive cycles: large, medium and small scale. In another, smaller one (Rankin M-36) can be recognize two cycles: large and medium scale.

Description of cycles:

The large-scale cycles thickness varies from 16 to 60 m. Boundaries are determined by layers of low gamma ray. On the southern boundary of the basin low gamma layers are represented by marl/limestone, on the northern-east by siltstone and sandstone. The large-scale system are composed of medium-scale cycles with a thickness varies from 4 to 15 m and whose barriers are also determine low gamma ray layers. The medium-scale cycles can be divided into small-scale cycles. 

Examples of occurrence  of cycles in particular wells:

Rankian M-36 (Fig. 6)

At this well the Egret Member possess four large-scale cycles. Each of them consist of four medium-scale cycles.

Archer K-19(Fig. 6)

At Archer K-19 the Egret Member contain four large-scale cycles which in further division consist four medium-scale cycles. Each medium-scale cycle possesses four to six  small-scale cycles.

Fig. 6  Sedimentary cycles obtained by mirror dispal of the gamma ray log. Rankian M-36, Archer K-19. A, B, C, D define arge-scale cycles. A1, A2, A3, A4 define medium-scale cycles (from Huang et al., 1996).

The relationship among the large, medium and small-scale cycles observed in others wells is similar.

Generally the ratio of:

-  large-scale / medium-scale cycles is 1:4

-  medium- scale / small-scale cycles is 1:5

Correlation of cycles

Four large-scale cycles can be single out (A, B, C, D). Every large-scale cycle contain medium-scale cycle (1,2,3,4).

The large-scale cycles are laterally continuous and can be correlated between wells. However thickness of this type of cycles varies laterally. The changing thickness is related to different depositional environments of the Egret Member, compaction and probably local erosion.

The medium-scale cycles are different in thickness laterally and vertically, but can be also correlated with others wells.

The small-scale cycles cannot be correlated between particular wells.

Example of correlation of the Egret Member are shown in Figure 7. 

Fig. 7 Correlation of sedimentary cycles in the Egret Member in Egret K-36, Rankin M-36 and Hibernia K-18. (from Huang et al., 1996).

Estimation of the duration of cyclicity

Although the laterally and vertically thickness variability of cycles in wells, the structure of the cycles shows that they represent record of some periodic geological process.

The relationship between three orders of cyclicity and their time ranges suggest that the depositional cycles are related with climatic and oceanic changes caused by orbital forcing (the Milankowitch cycles).

The cyclicity of large-scale cycles was calculated to be about 413 ka and suggest eccentricity cycle. The medium-scale cycles is about 100 ka and suggest also eccentricity. The small-scale  is about 20 ka and can be interpreted to have precession cyclicity (Fig. 8). 

Fig. 8 Range of estimated cyclicity of the three orders in the Egret member. E 1, E2, E3 - eccentrity cycles. P 1, P 2 - precession cycles (after Huang et al., 1996).

To sum up the Egret Member rock can be explained by orders of sea-level and climate fluctuations which were orbitally forced.

Sedimentation rates in the Egret Member

 

Fig. 9 Sedimentaion rate in (cm/ka)  for the Egret Member in the Jeanne d'Arc Basin (from Huang et al., 1996).

The estimated sedimentation rate of the Early Kimeridgian time in the Jeanne d’Arc Basin varies from 3,8 cm/ka (Ratkian M-36) to 14,7 cm/ka (Fortune G-57). The deposite of the Egret Member in three wells have higher sedimentation rate- 10 cm/ka (Archer K-19, Fortune G-57, Terra Nova K-18) and it is probably evidence of deltaic condition in this zone (Fig. 9).

The sedimentation rate was changing during the time which shows figure 10. This variation can by probably caused by rapid transgressions followed by gradual regressions. It can be worth to observe C3 and C4 fractional thickness. Decreasing of sedimentation rate in this cycles suggest increasing sea level. It also indicates that subsidence rate was constant. The smallest fractional thickness occurs in C3 may represent condensed section which popular occurs in rapid transgressions. Higher sedimentation rates are connected with lower sea periods (decreased accommodation).

Fig. 10 Hibernia K-18 A vertical variations in sedimentation rate B vertical variations in medium-scale cycle's thickness (from Huang et al., 1996).

Relationship between sedimentation rate variation and TOC content is worthy of note and it is the last aspect which I want to bring up in this note.

Figure 11 shows interrelationship between sedimentation rate and amount of TOC.  Huang et al. (1994) interpreted this as amount of  TOC accumulation is connected with sea-level and climatic change which has a reflection in sedimentation rate.  High amount of organic matter is related with small sedimentation rate. Factors which caused enlargement of accumulation of organic matter during Kimmeridgian are: transgression, warm climate, inhibited oceanic convection and anoxic near the sea floor. The Egret Member rocks are developed as laminated, brown shales, which represent anoxic conditions. A rising of sea level and warm climate created favorable conditions to deposition and preservation of organic matter. Euhedral pyrite was detected in samples and it is next evidence of existence of anoxic water. Marlstone/limestone and unlaminated claystone which also belong to the Egret Member, represent more oxygenated water (connected with lower sea-level) and decreased organic matter accumulation. During a period of low sea level organic matter was scattered in thick sediments coming from lands. Periods of higher sedimentation rates caused quick burial of organic-rich beds. 

Fig. 11 Egret N-46 (see text for explanation)(from Huang et al., 1996).

Described factors are very important for understanding why the Egret Member is so interesting not only from economical point of view.

Bibliography

Fowler M. G. & McAlpine K. D., 1994. The Egret Member, a prolific Kimmeridgian source rock from offshore Eastern Canada. In: Petroleum Source Rock Case Studies (Ed. B. J.Katz), Springer, Berlin, 111-130.

Huang Z. et al., 1996. Cyclicity in the Egret Member (Kimmeridgian) oil source rock, Jeanne d'Arc Basin, offshore eastern Canada, Marine and Petroleum Geology, Vol. 13, No.1, 91-105.