A Petrographic Study of the Three Forks Formation (Upper Devonian), Williston Basin, North Dakota: Based on Thin Section Analysis, XRD and SEM

Deeply buried below 8,000 ft, theThree Forks Formation in North Dakota displays a variety of interesting diagenetic characteristics including dolomitization and hematite precipitation. Samples from three lithofacies are analyzed by thin section and SEM petrography and combined bulk and clay XRD analyses. Key aspects controlling the porosity and permeability of this formation are revealed by focusing on the detail mineralogy, rock type and diagenetic mineral distribution, and overall reservoir quality. Results prove that the Three Forks mineralogy is dominated by dolomite, along with substantial hematite, monocrystalline quartz, and mica flakes with trace feldspar, calcite, and pyrite. Clays mainly consist of illite together with minor chlorite and kaolinite and are associated with the scattered clasts.The reservoir quality is controlled by intercrystalline, rare microvuggy, andmicroporosity types that result from diagenetic and depositional events. Three stages of the dolomitization process are identified and discussed. Our hypothesis is that dolomitization commenced soon after deposition andwas pervasive as no original carbonate texture is detectable.


Introduction
The Three Forks Formation exists within the Williston Basin, occupying nearly two-thirds of North Dakota, underlying the Lower Mississippian Bakken Formation, and overlying the Upper Devonian Birdbear Formation (Figure 1).The Three Forks Formation is one of the most unpredictable petroleum systems within the Williston Basin with a combination of carbonates, clays, organics, and detrital material of poor to moderate reservoir quality, reflecting its depositional and postdepositional fabric.This study serves as an attempt to provide critical information on the prospectivity, in terms of key diagenetic processes that operate within the Three Forks, based on a derived dataset of results from thin section analysis, X-ray diffraction (XRD), and scanning electron microscopy (SEM).The aims of this study are to determine the following: (1) mineralogical composition, (2) differences in texture, (3) framework fabric, (4) sequence of diagenetic events, (5) pore system, and (6) reservoir quality of the Three Forks in three identified lithofacies [1].Close-up photomicrographs are provided to stress (1) preserved primary and secondary pore types, (2) cements, (3) sedimentary structures, (4) fractures, and (5) pore plugging hematite.A brief summary of sample information, rock type, lithology, thin section porosity, analysis types, original texture, and reservoir quality are presented as Table 1.Results exemplify the need for a more comprehensive study as much is still undecipherable.It is hoped that this study will fuel further, targeted multifield, and regional studies to provide insight into the flow-controlling reservoir heterogeneities that increasingly challenge the drive to optimize hydrocarbon recovery within the formation.

Materials and Methods
The data and interpretations for this study were based upon a detailed petrographic examination of core samples from three      facies are chosen to obtain maximum representation.Small portions of fresh samples were analyzed by bulk and clay XRD techniques using the fingerprinting approach [3,4].Six (6) XRD samples (two from each facies) are taken from the same position as the thin section samples.Thin sections are prepared following standard petrographic procedures to a size of 30 m.Samples had been impregnated with blue epoxy and stained with a combined carbonate stain of Alizarin Red-S solution and potassium ferricyanide [5,6].Visual estimates of grains and cements are obtained by point counting (300 points).Small pieces of each sample are examined by SEM to highlight the occurrence, distribution, and origin of micropores, plus constituent minerals and their textural features.Energy dispersive X-ray (EDX) analysis is also carried out to get the elemental composition of the samples.

Results and Discussion
3.1.The Bulk and Clay XRD.Results of the combined bulk and clay XRD analysis are presented as in Table 2.The nonclay portions of the samples are dominated by dolomite (56% to 60%) along with lesser quartz (14% to 16%), with minor amounts of potassium feldspar (2% to 3%) and plagioclase feldspar (1% to 2%), calcite (1% to 2%), hematite (trace to 1%),  and trace siderite.The clay portion consists of illite (14% to 15%), together with chlorite (2% to 4%) and kaolinite (3%).The clay fraction (<3 m fraction that best represent finer matrix clays) ranges from 4.85% to 8.73% of the total weight volume of the samples and consists of (76% to 86%) clay minerals.Pore-filling kaolinite is present in all samples and together with chlorite and illite is interpreted as terrigenous, weathered products of plagioclase and K-feldspar.

Thin Section Analysis and Interpretation.
The thin section petrographic summary is given in Table 3.Three main stages of dolomitization are observed in these samples.The first stage or dolomite type 1 (Dol 1) is classified as very fine to fine crystalline (<62.5 m) generally anhedral and unimodal dolomite consisting of tightly to loosely interlocked crystals (Figures 3-5) that replace the matrix of the original rock.Intercrystalline porosity occurs in poor amounts in dolomite type 1.The second stage of dolomitization (Dol 2) which replaces the earlier formed dolomite is the most abundant (26% to 35%) and generally better formed subhedral to euhedral dolomite rhombs (>62.5 m or medium crystalline) that cement the larger intercrystalline pores (Figures 3-5).Also, this dolomite occasionally presents a sucrosic texture that may have resulted to a certain degree from the bulk volume shrinkage that accompanies calcite replacement by dolomite or from the dissolution of residual calcite during the final stages of dolomitization [7].Rare medium to coarsely crystalline euhedral dolomite cements are considered last stage (3rd generation) dolomite cement and are present mostly in the porous areas (Dol 3) consisting of dolomite rhombs (1% to 3%).These dolomite rhombs often have saddle morphology and cement the intercrystalline pores (Figures 4 and 5).Both the 2nd and 3rd generation dolomites are occasionally turquoise stained, highlighting the presence of iron rich dolomite.

Lithofacies 1.
This facies has a thin section porosity of 3%.At moderate magnification view, this facies consists of very fine to medium crystalline dolomite rhombs (Figure 3(a)).Monocrystalline quartz and trace amounts of feldspar grains are the detrital sand grains that are scattered throughout.Overall, the replacement dolomite is dominated by anhedral to subhedral interlocking very fine crystalline dolomite rhombs, with substantial amounts of recrystallized fine crystalline 2nd stage dolomite and lesser late stage void rimming fine to medium crystalline euhedral rhombs.The mineralogy of this sample is dominated by dolomite (54%) with moderate amounts of matrix material (20%), hematite (13%), and quartz (10%) along with minor muscovite (Mica, 3%), plus trace calcite, feldspar, and pyrite.Moderate hematite and trace pyrite are the noncarbonate diagenetic minerals that substantially occlude pores.Hematite also occurs as microblebs and replaces the clay matrix and the anhedral-subhedral dolomite rhombs.Visible porosity is represented by small size isolated minor intercrystalline (2%) and microvuggy (1%) pores.Microfracturing is quite common in this facies and may probably result from unloading fracture (Figure 3(a)).Figures 3(b)-3(d) highlight pervasive dolomitization which appears to have created all the visible small isolated intercrystalline pores.Dolomite rhombs are in places slightly ferroan.The hematite is interpreted as the final weathering product of pyrite or siderite formed during the last diagenetic step [8].Minor amounts of clasts (clay and organics) generally occur as lenses and create permeability barriers.5).Similar to facies 1 and 3, detrital sand grains consist of monocrystalline quartz and trace amounts of feldspar but the replacement dolomite s dominated by ferroan and nonferroan, subhedral recrystallized 2nd stage dolomite (Figure 5(a)).This facies also contains more hematite than the others.Mineralogically, this facies is dominated by dolomite (40%) and hematite (37%) cements with moderate amounts of matrix material (16%) and lesser quartz (5%) along with minor muscovite (2%), plus trace calcite, feldspar, and pyrite.The original carbonate texture would have been mudstone.In this facies hematite cement appears to have occluded all the intercrystalline pore spaces.Long, thin detrital grains of mica (Figures 5(b) and 5(d)) are also present.

SEM Analysis and Interpretation
SEM reveals that authigenic hematite is mostly small, round, or disc shaped, and coarsely crystalline (Figure 6).The coarser hematite occurs either as irregular platy crystals or as well-developed hexagonal crystals (similar in appearance to kaolinite crystals) and is dark red to black in reflected light.Dolomite shows abraded subhedral, rhombic termination which is strongly suggestive of resedimentation [9].Scattered hematite inclusions within these abraded dolomite rhombs and detrital quartz grains are responsible for the reddishbrown coloration of these facies.The EDX spectra illustrating dolomite and hematite cement show that the major elements of hematite are primarily iron (Fe) and that of dolomite as (Ca, Mg), with additional Si, Al, and K from the detection of adjacent quartz grains, plus illite and kaolinite clays.

Diagenetic History
Diagenetic features, in a paragenetic sequence, include onset of compaction, precipitation of calcite and pyrite, dolomitization or dolomite cement in pores, and hematite precipitation (Figure 7).Compaction of the sediment sets in immediately after deposition and increases during burial due to rising lithostatic stress.Early stage of compaction reduces volume and porosity of sediment by expelling interstitial waters and framework becomes slightly compacted.Precipitation of pyrite occurs in two stages.The first stage pyrite precipitated in early reducing conditions, occluding pores and replacing matrix clays while the later stage pyrite is locally precipitated into the earlier formed dolomite rhombs.We therefore hypothesize that pervasive dolomitization, caused by the influx of magnesium rich fluids, took place shortly after deposition.The dolomite 1 mimics the original finegrained limestone during replacement resulting in a very fine to finely crystalline texture.Euhedral dolomite cement

Figure 1 :
Figure 1: Extent and stratigraphic position of the Three Forks Formation showing members 1 to 5, the Pronghorn Member (P), and sample locations.

Figure 2 :
Figure2: Selected lithofacies for petrographic analysis, from the top, middle, and lower Three Forks.

Facies 1 A 3 A 4 AFigure 6 :Figure 7 :
Figure 6: SEM photomicrographs of the Three Forks showing the distribution of very fine to medium crystalline dolomite rhombs (Dol) and small, disc shaped authigenic hematite cements (H) in all samples (a, b, c).The intercrystalline (Ixl, A: E4, N5) porosity is mainly obstructed by the precipitation of anhedral to subhedral dolomite rhombs and subordinately by hematite cements.
Facies 3 consist of very fine to medium crystalline dolomite rhombs and monocrystalline quartz and feldspar detrital sand grains (Figure 4(a)).Photomicrographs (Figures 4(b)-4(d)) highlight the replacement dolomite in this facies, dominated by ferroan and nonferroan anhedral to

Table 1 :
Summary of sample information.

Table 2 :
Summary of Bulk and Clay XRD Analysis.Bulk and clay: mathematical recalculation including the bulk and clay fraction representing the whole sample.Total clay: sum of the clay minerals (may include authigenic and matrix clays plus clays in rock fragments).

Table 3 :
Thin section petrographic summary of three dolomitic samples from Facies 1, 2, and 3 of the Three Forks Formation.