Middle Triassic Halfway FormationAssessment of the Dimsdale Oil

Field

Arsalan Syed

Taylor Harris

Basin Evolution

This project entails studying the Halfway Formation south of the Wembley Field

The Halfway formation was deposited during the Middle Triassic in the Western Canadian Sedimentary basin, it occurred in the shallow epicontinental seaway that covered much of western Canada at the time. (Cant, 1986)

This sea was very prone to transgressions and regressions due to its shallow yet wide basin. The Halfway is deposited within what is known as the Peace River Embayment; during the Carboniferous period the Peace River Arch (PRA) was transformed into a graben complex known as the Dawson Creek GrabenComplex.

This wide low relief embayment persisted through the Permian into the Triassic, subsidence was similar to that of the Permian, and was home to the deposition of the Halfway Formation.

The Halfway formation lies above the Doig formation, which consists of phosphorous siltstones and shales. The top of the Doig is a transgressive unconformity, leaving a basal lag.

The Halfway formation sits on top of this transgressive unconformity and is primarily sandstone, with occasional siltstones and shales towards the base.

Willis & Moslow, 1994b

Project Study Area

Willis & Moslow, 1994b

Basemap of Study Area

Depositional Environment The Halfway Formation is primarily composed of sandstone that are generally

laterally extensive through the unit. Starting from the Northeast the Halfway is usually only about 5 meters thick, further west continuing out through the embayment the unit thickens to over 85 meters to the southwest (Campbell et al., 1986) because of major basin subsidence towards the middle of the Western Canadian Sedimentary Basin.

Sediment is driven east and southeast by longshore drift winds. Paleoclimate in the area was likely less than 25 cm of annual precipitation, and up to 2 m of annual evaporation. (Campbell et al., 1986).

During times of shoreline progradation, wave dominated deltas formed, leaving transgressive barrier islands, and strandplain shoreface successionsBehind these are arid environments, including hypersaline lagoons, sabkhas, and tidal flats but these units are not found in the Halfway Formation.

The barrier island, tidal inlet sequence that makes up the Halfway can be described by two facies that often occur together but are somewhat distinct. They are described by shoreface and remnants of barrier islands overlapping offshore deposits, and tidal inlet fill sequences.

Depositional Environment Two different facies standout in our area, the first being a coarsening-upward succession of

interpreted to be offshore shales and siltstones which because of the shallow low subsidence in the basin leads to progradation of the barrier island shoreface onlapping the offshore in thin continuous manor.

This progradation leaves a thin shoreface sandstone approximately 5-6 m thick through our section. When transgressive barrier islands are preserved with these, they sit on top of the shoreface deposits and increase the thickness of the sandstone body and extend laterally with the strike of the shore.

The second facie is a tidal channel and inlet fill sequences. Between the barrier island, tidal inlets cut through the shoreface, they often leave shell-fragment hash and channel sandstones. The tidal influences are likely microtidal farther to the west to mesotidaltowards the east of the Halfway.

They are generally not continuous along shoreline strike but are more continuous along shore dip (southeast-northwest), generally not longer than 3-8 km’s or wider than a few km’s.

As well, because of the mesotidal influence of the area the barrier island shift and move about, leaving many tidal inlet infills along depositional strike. These tidal inlets can even coalesce or stack on top of each other and form good hydrocarbon plays such as the Wembley Field.

Gross Sand Isopach

Campbell & Horne, 1986

Bubble Map of Hydrocarbons

Stratigraphic and Structural Cross Sections

A-A’ Cross Section

B-B’ Cross Section

A-A’ Stratigraphic Section

B-B’ Stratigraphic Section

Trapping Mechanisms The Middle Triassic Halfway formation has many trapping mechanism that can hold

hydrocarbons. The main trapping mechanism inside this continuous sand body are facies-change pinchouts or erosional pinchouts of reservoir units (Halton, 1981). There is a complex relationship between the different forms of stratigraphic traps that occur within this unit.

Two main types of reservoirs are apparent, tidal inlet channel infill reservoirs and transgressive barrier island reservoirs

The sandstone and shell-hash fill tidal inlet channels are laterally discontinuous along the shoreline, and longer along shore dip. These create structures with in the sandstone body that are better for holding hydrocarbons than the shoreface as they are generally thicker and discontinuous, creating minor pinchouts.

Varying amounts of porosity and permeability due to the variations in diagenetic dolomitizationof the sandstone and shell-fragment hash. In some cases the shell fragments are dissolved away increasing porosity, and in other cases permeability is decreased by the dolomite cement between grains. In addition to this, post-dolomitization anhydrite, has also filled pores further decreasing porosity (Cant, 1986).

This varying porosity and permeability within these units makes every tidal inlet fill different and often limits the net pay within them to a few meters. In other cases this doesn’t affect the reservoir heavily and leads to a good reservoir, like the collection of inlet fills that make up the Wembley field to the north of our study area.

Trapping Mechanisms

The varying changes in sea level during the deposition of the Halfway Fm lead to highly variable sedimentary structures within the unit as many parts of the shoreface become eroded.

There is low preservation of transgressive barrier islands, as they just become reworked by the regression towards the sea. Although poorly preserved, some, depending on how the minor transgressions occur, small amounts of the barrier islands can be preserved and sit on top of the shoreface. They tend to be opposite of the inlet fills by being longer bodied along strike than dip.

The barrier island facies also suffer similar diagenetic alteration as the inlet fill units, so porosity and permeability is variable. These can make good traps within our formation, as a unit that is closed by being a stratigraphic high and able to hold hydrocarbons.

Willis & Moslow, 1994b

Source Rocks The possible source rock for the hydrocarbons trapped in the Halfway is

hypothesized to come for the Doig and Montney Formations (Riediger et al., 1990).

Both the Doig and Montney were deposited in an open marine shelf in relatively deep marine conditions (Gibson and Barclay, 1989; Riediger et al, 1990) and Triassic transgressions are known to leave organic rich shales.

The phosphatic zone of the Doig was found to produce type II kerogen with TOC varying from 2%-11% whereas the upper part of the Doig predominantly produced kerogen of type II/III, with TOC values lower than 2% (Riediger et al., 1990).

This type of kerogen would account for the findings of mainly gas with some oil within our section of the Halfway. The phosphatic zone of the Doig has good to excellent hydrocarbon source potential and are the most likely the source for the Halfway formation.

Volumetrics For volumetric assessment the Dimsdale Halfway B field was analyzed.

This field contains three wells in which two wells contain gas (100/08-28-071-07W6) and one well contains oil (100/08-28-071-07W6).

The wells with gas have a gas pay top of 2162.4 m KB and a gas pay bottom of 2164.3 m KB. The area of the wells is 1067 ha. The pay thickness is 1.5 m with porosity of 9.1% and gas saturation of 70%. The original gas in place (OGIP) was 121798.88 m3/ha-m and the recovery factor was 73.5% which resulted in a cumulative production of 164.89 e6 m3. These wells produced gas between June 1981 to March 2013. The OGIP remaining is 27% or 1.11 e6 m3.

The well with oil has an area of 64 ha and a pay thickness of 4.5 m. The porosity within the pay thickness is 7.3%. The density of the oil produced is 820 kg/m3. The original oil in place (OOIP) in this well is 285 m3/ha-m. The recovery factor is 31.5% which resulted in a cumulative oil production of 24.78 e3 m3 between 1981 to 2013. The total OOIP remaining is 70% or 1.12 e3 m3.

Conclusion The Middle Triassic Halfway Formation is one of the most significant producers of oil and gas

within the Triassic in western Alberta and eastern British Columbia.

As a laterally extensive sandstone body ranging from 5 m to over 85m thick, the formation has a complex internal relationship. Influences of longshore drift, macro and mesotidal influences and numerous transgressions and regressions have shaped and formed the Halfway Formation.

The Halfway is primarily composed mainly of shoreface bedded sandstones sitting on top of offshore siltstones and shales, with inlet tidal channel infills and transgressive barrier islands along depositional strike.

Beneath the Halfway is the organic rich Doig Formation, which is likely the hydrocarbon source, producing gas and oil to be trapped within the Halfway. Oil and gas production is common within tidal inlet fills and transgressive barrier island complexes.

Trapping of hydrocarbons occurs within stratigraphic traps formed by these structures as well as within pinchouts further east out of our study area.

The study area analyzed in this report contains several preserved barrier islands and two significant tidal inlet channels.

The majority of the study area contains gas reservoirs with a few wells found in the tidal inlet regions producing oil.

The Dimsdale Halfway B oil pool was analyzed and contains three wells in which two produce gas and one produces oil. This field was drilled for hydrocarbons between 1981 to 2013 but there is still 70% of oil in place and 27% of gas in place. We suggest re-visiting this oil pool and continue drilling for further hydrocarbons since the oil and gas recovery rates are good. The rest of our area contains majorly gas which would not be economically efficient to further explore or drill.