Figure 3-7 Stratigraphic Correlation of FANNY_18B_17-FANNY_18B_11
3.1.3 Identification of the key surfaces.
(1) Identification of the top of S1 cycle.
The S1 cycle is related to falling periods of a medium-term base level cycle. The main lithology types are
coal and peat. Sandstone deposition is very limited in this interval. The top of the S1 is the transfer
surface between base level rising and falling. The top of the S1 is recognized as the top of the coal be or
the base of the thick overlying sand if the coal bed is missing (Fig.3-8).
(2) Identification of the top of S2 and S3.
The S2 and S3 periods sand intervals belong are characteristic of a rising period of a medium-term base
level when base level was relatively low. Sandstone is well developed in most of the area except in the
area represented by cross sections Dorine_24—Dorine_27—Dorine_62—Dorine_66—Dorine_19 and
the section Fanny_18B_12—Fanny_18B_80 — Fanny_18B_72, where mudstone (shale) is the dominant
lithology.
Figure .3-8 S1 Surface Identification Column Maps
In most of sandstone regions, the GR curve is characterized as box shaped with relatively low values.
Most resistivity curves show the box shape with high values (Figure 3-9 and Figure 3-10) The shape of
the resistivity curves is box-like because they are easy influenced by the oiliness, physical properties, and
lithology of the layer.
Figure 3-9. S2 and S3 Surface
Identification Column map
Figure .3-10 S2 and S3 Surface
Identification Column Map
The lithology of the layer below S3 is gray shale with lenticular bedding and thin carbonaceous beds.
Chondrites and Platycopa fossils are found in this interval.
A medium-fine sandstone, with swaley cross stratification and tabular cross stratification is immediately
above the base of S3.
A black carbonaceous mudstone and coal is just above top of the S2.
The Lithology and logs are varied below S3 and above S2. This is the interval where sandstone is well
developed, so they can be easily identified. These two boundaries are defined as the half amplitude point
of the SP and the GR curves, and mid-point of the LLD and LLS curves (Figure 3-11).
Figure 3-11. S2 and S3 Surface
Figure 3-12. S2 and S3 Surface
Identification Column map
Identification Column map
In the area where a thinly bedded argillaceous sandstone developed, the character of the bottom interface
of S3 is the same as in the sandstone region.
Comparison of the bed thickness with adjacent wells and the sequence stratigraphy are key to picking the
top and bottom boundaries of S2.
During deposition of the S2 and S3, sediment base level oscillated in the lower cycle. The bottom
interface of S2 is supposed to be divided at the maximum position of the base level. However we decided
to pick it at the half amplitude point of the GR curve, and where the position of the LLD and LLS curves
become discontinuous for the sake of analyzing the interbeds as discussed below (Figure 3-12).
In order to describe the inter-bedded layers, we divided the S2 and S3 into 5 secondary cycles: S2-1, S22, S3-1, S3-2, and S3-3. We did this based on the base level cycle, the sedimentary sequences, and the
thickness of the layers and character of the logging curves,
3.2. Barrier Layer and Interlayer Analysis
3.2.1 Types of Interlayers.
Barrier layers, also called shielding layers or seals, are low permeability or impermeable intervals that
constrain or prohibit flow across their surfaces. Barrier layers are distributed throughout the reservoir a at
scales ranging from tens of centimetres to several decametres. Interlayer is a term assigned to the
relatively low permeability to impermeable layers in sandstone that are found intermittently and at scales
from several centimetres to tens of centimetres. Interlayers influence flow of the reservoir at small scales
but usually do not effectively constrain the reservoir-scale flow.
Both barrier layers and interlayers are classified lithologically as mud-based or carbon-based. Mud-based
layers are mudstone or silt-mudstone, have low oil saturations, and usually belong to tidal valley, delta
front, or tidal creek microfacies. Carbon-based barrier layers and interlayers contain coal and/or peat
layers, and usually belong to mud flat microfacies of the delta plain. For the study area the porosity must
be less than 10% for a bed to be classified as a barrier layer or interlayer.
Barrier layers and interlayers are differentiated by aerial extent and thickness in the work area. Mud or
carbon-based layers thicker than 15 ft. and wider than 80% of working area are called barrier layers; those
thinner than 10 ft. and covering less than 30% of the working area are called interlayers. Barrier layers are
distributed through the S1 interval. In the following analysis, we call barrier layers interlayers when it is
not necessary to make the distinction.
3.2.2 Interlayer Analysis.
During S3-3 deposition, base level was at its lowest, layer thicknesses changed quickly and few layers
extended over wide areas. During this time mudstone deposition was not common with just over 0.5 ft
found in the Fanny–18B–20 well (Figure 3-13).
During S3-2 deposition, continuous mud-based interlayers were distributed in two NW-SE trending strips
in the eastern part of the area, around a line connecting Dorine 25–ST1, Dorine–49, Dorine–62, Dorine–
66, Dorine–19, and a line connecting Fanny–18B–12, Fanny–18B–80, Fanny–18B–70, and Fanny–18B–
73. Another strip (averaging 8.47 ft. in thickness) is located in the southern part of the area, around a line
connecting Fanny–18B–69, Fanny–18B–70, and Fanny–18B–73(Figure 3-15). Both strips are visible in
seismic amplitude slices (Figure 3-14).
In the S3-1 interval the distribution of mud-based interlayers is similar to S3-2. In the east, the average
thickness of two mud-based zones is 12.8 ft and 8.5 ft., respectively. In the north, mud-based interlayers
are wider than in the S3-2 (Figure 3-16).
Mudstone sealing
FANNY_18B_12
FANNY_18B_69
FANNY_18B_70
FANNY_18B_97
Figure 3-13 Interlayer Contour Map of S3-3
Figure 3-14 Seismic Amplitude Slice Map
In the S2-2, mud-based interlayers are thicker and more widely distributed. Their distribution is similar to
that of the S3-2 and S3-1. The mud-based stripe is well-developed in the east. The average thickness of
mudstone is 9.77 ft. In the south there is a mudstone region 14.5 ft. thick near the Fanny–18B–97 well. In
other regions, however, the spatial distribution of mud-based interlayers is different for different subfacies
(Figure 3-17).
In the S2-1 the distribution of mud-based interlayers is very similar that of the S2-2, and the thicknesses
are just a little smaller than for the S2-2 (Figure .3-18).