Study of Hydrocarbon Potentials and Sedimentary Properties of Ispartaçay Formation, Turkey

Abstract


Introduction
The study area is located in Antalya Nappes area south of Isparta City, southwest Turkey (Fig. 1).This zone shows complex features including two autochthonous rock units (Anamas-Akseki platform and Beydağları platform) is closed by the allochthonous rock units (Antalya Nappes, Lycian Nappes, and Beyşehir-Hoyran Napps).According to Özcelic et al. (2015), the Antalya Nappes area includes approximately three kinds of subunits: (i) ophiolites subunit (mainly peridotites and gabbros), (ii) shallow-marine platform carbonates subunit, (iii) basinal sequences within, in places, volcanic intercalations subunit (Vrielynck et al., 2003) (Fig. 2).(Yağmurlu et al., 1997) Ispartaçay Formation was deposited during the Cretaceous and was described for the first time (Guting et al., 1979) in the Isparta which is comprised of thick massive, cherty limestone and shale.According to Yavuzlar (2015), the formation has been long-ranging from the Early to Late Cretaceous.The Ispartacay Formation was subjected to sufficient burial to start oil generation then it was uplifted, where the studied area was affected by duplicated thrust many times and uplifted up (Yavuzlar, 2015).The main aim of this research is to determine the quality and quantity of organic matter, hydrocarbon generation potential, and maturity of the organic matter in the study area, in addition to identifying the petrographic features, microfacies analysis, and sedimentary environment of the Ispartaçay Formation.

Fig.2.
A geological map of the Isparta city and adjacent areas (Vrielynck et al., 2003)

Materials and Methods
Ispartaçay Formation in the south of Isparta City with a thickness of 200 m was chosen for the study of this research (Fig. 3).150 samples were collected from this formation according to the change in the stratigraphic, sedimentary, and organic features.In the laboratory of Süleyman Demirel University, thin sections were prepared and used in the petrographic study.In addition, total organic carbon analyses, and Rock-Eval pyrolysis were undertaken at the Turkish Petroleum Research Center (Turkey).

Investigation of Source Rock Potential
About 24 samples were selected for analysis from 150 samples collected from the Ispartaçay Formation (Table 1).The selection of samples was mainly based on rocks containing organic matter, to determine the amount and type of organic matter, and the level of thermal maturity of the organic matter in the formation.The TOC contents of Ispartaçay Formation samples range from 0.14 -1.82 wt% with an average of around 0.88, S1 ranges from 0.01 -0.57with an average of around 0.13, S2 ranges from 0.11 -1.02 with an average of around 0.43, and PP ranges from 0.13 -1.57with an average of around 0.62 (Table 2).The TOC results are based on the classification of Peters and Cassa, (1994) and PP on the classification of Tissot and Welte, (1984), indicating that this formation has a fair potential for organic matter as the source of rocks for petroleum generation.
Table1.The Total Organic Carbon (TOC), and Rock-Eval pyrolysis results for the Ispartaçay Formation TOC: Total Organic Carbon, wt%.S1: Free hydrocarbon content, mg HC/g rock.S2: Hydrocarbon generated from the thermal breakdown of kerogen, mg HC/g rock.S3: Carbon dioxide value, mg CO2/g rock.Tmax: Maximum temperature.HI: Hydrogen Index= S2/TOC×100, mg HC/g.OI: Oxygen Index= S3/TOC×100, mg CO2/g.S2\S3: Hydrocarbon type index.PP: Petroleum Potential= (S1+S2).Ro: Calculated theoretically vitrinite reflectance value according to the equation of Jarvie et al. (2001) Based on the classification of Peters and Cassa, (1994).Ispartaçay Formation sediments or rocks are characterized by a hydrogen index (HI) ranging from 8.5 (equivalent to type IV kerogen) to 180 (equivalent to type III kerogen) with an average of around 65.25 mg HC/TOC, in addition to S2/S3 range from 0.0 (equivalent to type IV kerogen) -2.70 (equivalent to type III kerogen) with an average of around 0.79 (Table 3).
The organic geochemical data of the 24 samples selected for rocks analysis of the Ispartaçay Formation were plotted on a TOC-S2 diagram according to (Peters and Cassa, 1994 ;Ghori, 2002) to describe the quantity and potentiality of organic matter (Fig. 4), HI-OI diagram according to (Espitalie et al., 1985) to show the quality of organic matter (kerogen types) (Fig. 5), and HI-Tmax diagram according to (Espitalie et al., 1985) to show the maturity of organic matter (Fig. 6).
Fig. 4, which indicates the fair to poor hydrocarbon potentiality of that most of the Ispartaçay Formation samples as source rocks for petroleum generation.Fig. 5, which indicates assemblages of type IV and III kerogens for the samples formation.Fig. 6, the diagram indicates that most of these formation samples represent the post-mature (gas window) to mature (oil window) stage.Based on classification (Peters and Cassa, 1994;Hunt, 1996).Tmax values range from 432 °C (Immature) for very few samples to 532 °C (Post mature) for most samples, Ro values range from 0.62 (Early mature) for very few samples to 2.41 (Post mature) for most samples (Table 4), these results indicate that most of the Ispartaçay Formation samples are post-mature (gas window) to mature (oil window) stage.Fig. 6.HI versus Tmax plot describing the stages of organic matter maturation in the Ispartaçay Formation (Espitalie et al., 1985) The results of the current study were compared to previous studies as a study (Özcelic et al., 2015), and the results of the analysis of the quality and quantity of organic matter, hydrocarbon generation potential, and maturity of the organic matter were identical to the current study.

Microfacies Analysis and Paleoenvironment
The prepared thin-section characterizations of samples from the studied area were used to define two depositional microfacies.The classification used here is the Dunham (1962) classification, depositional microfacies are identified below and are correlated with the corresponding microfacies kinds from Wilson's (1975) microfacies models which were modified from Flugel (2004).

Lime mudstone microfacies
These microfacies are the widespread facies in the study area and it consists majorly of mud as groundmass and little radiolarians, planktonic and benthonic foraminifera.Lime mudstone microfacies consist essentially of micrite which is lightly affected by diagenetic processes like silicification compaction, cementation, and dissolution.Lime mudstone microfacies include two submicrofacies:

Radiolarian lime mudstone submicrofacies (IS1)
IS1 submicrofacies are represented by micrite as a matrix (higher than 90%) with a few particles as radiolarian, planktonic and benthonic forms (less than 10%) (Fig. 10A).The groundmass is characterized by dark color.The common diagenetic features in these submicrofacies are silicification and dissolution (Fig. 10B).These submicrofacies appear in the middle and lower parts of the Formation.According to Flugel (2004), these submicrofacies are equivalent to standard microfacies (SMF3), which belong to Facies Zones (FZ-2) outer ramp environment.

Planktonic foraminifera lime mudstone submicrofacies (IS2)
IS2 submicrofacies consist of micrite as groundmass (higher than 90%) with little of which included planktonic forms content (less than 10%) (Fig 10C).The matrix is characterized by dark color.The diagenetic features in these submicrofacies: are cementation and authigenic minerals.These submicrofacies appear in the middle and upper parts of the Formation.According to Flugel (2004), these submicrofacies are equivalent to standard microfacies (SMF4), which belong to Facies Zones (FZ-4) mid-ramp environment.

Lime wackestone microfacies
The skeletal grains ratio in this microfacies is equal to (15%) and represented by: radiolarians, planktonic-benthonic foraminifera, and bioclastic.Which are the major ones backed by mud.The diagenetic features identified in these submicrofacies: are compaction, cementation, and dissolution.Lime wackestone microfacies include each of the following submicrofacies:

Conclusions
The study revealed the following results: • When evaluating data and diagrams, the sediments of the Ispartaçay Formation are characterized by poor-fair amounts of organic matter.Most samples from the Ispartaçay Formation contain kerogen components (terrestrial and marine material) of type III and IV.In addition, most samples of the Ispartaçay Formation were distributed in a post-mature (gas window) to mature (oil window) stage.The succession of the Ispartaçay Formation consists of massive cherty limestone, thin radiolarian limestone, marly limestone, and thin beds from dark gray bitumen shale, these rocks refer to the mid-ramp to outer ramp environment which is a good place to form source rocks.• The major petrographic study of Ispartaçay Formation components has shown the reddish argillaceous limestone, chert, turbidity limestone, and dark gray bitumen shale, and the groundmass consisted of micrite with light brown-dark color with poor to fair content of organic matter.In addition, this formation has Radiolarian fossils and a little ratio of foraminifera.Different types of diagenetic features were identified in the successions of the Ispartaçay Formation like silicification, cementation, compaction, and dissolution.

Fig. 3 .
Fig. 3. Stratigraphic successions of Ispartaçay Formation at the study location

Table . 2
. TOC and PP values for the selected samples from selected from the Ispartaçay Formation

Table . 3
. S2/S3 and HI values for the selected samples from the Ispartaçay Formation

Table 4 .
Tmax and Ro values for the selected samples from the Ispartaçay Formation (Peters and Cassa, 1994 ;describing the quantity and potentiality of organic matter in the Ispartaçay Formation(Peters and Cassa, 1994 ; Ghori, 2002)