Integrated petrophysical analysis and petroleum system modeling to assess the effect of igneous intrusions on source rock maturity enhancement and hydrocarbon potential in the Wichian Buri sub-basin, Thailand

Abstract

The Wichian Buri sub-basin is located in the southern Phetchabun basin, Thailand. It is reported to have significant recoverable hydrocarbons from relatively shallow fractured igneous rock reservoirs, but until now no detailed research has been done on the effect of the igneous intrusion on thermal enhancement of maturity of the source rocks and their hydrocarbon generation. The particular feature of the petroleum system in this basin is that the igneous intrusions of 11–16 Ma and 18–24 Ma played a dual role. On the one hand, they increased maturity of the source rock, and on the other hand, they became shallow fractured igneous rock reservoirs. In this study, well logging analysis and 1D petroleum system modeling (1D PSM) were integrated to evaluate the source rock of the Wichian Buri sub-basin based on the data from public domain of the petroleum industry of Thailand, including vertical well sections, well logs, regional geological sections, base maps, and geochemical data. The results of well log analysis found intervals of fractured igneous rock reservoirs with typical low gamma ray of 20 to 30 API and average porosity around 8 to 12%. The well log data were used to determine the total organic carbon (TOC) by the ΔlogR technique that was further used in 1D petroleum system modeling (1D PSM) with consideration of the effect of igneous intrusion on enhancing source rock maturity. Vitrine reflectance (Ro) and hydrogen index (HI) of the source rock in this study were found as 0.62% and 222 mgHC/gTOC, respectively. As a result of 1D PSM, hydrocarbon generation was found to have started in the Permo-Triassic Nam Duk Formation and ended in the late Miocene basalt and Miocene source rock. For an original hydrogen index (HI⁰) of 600 mgHC/gTOC, the hydrocarbons generated, retained, and expulsed in/from the source rock were estimated as 3178.6, 2823.0, and 355.5 MMSTB, respectively. By correcting HC_exp for petroleum system efficiency (PS_eff), which was chosen as 5, 8, and 15% in this study, one can obtain the charge volume of hydrocarbons that could be trapped as HCIIP, which is 53.33 MMSTB with PS_eff of 15%, indicating a good hydrocarbon potential for further exploration in the Wichian Buri sub-basin.

Appendix. Formula

The shale volume (V_sh) is calculated by Eq. 15, and the porosity is determined using Eq. 16–18.

$$ {V}_{\mathrm{sh}}=\frac{\mathrm{GR}-{\mathrm{GR}}_{\mathrm{sd}}}{{\mathrm{GR}}_{\mathrm{sh}}-{\mathrm{GR}}_{\mathrm{sd}}} $$

(15)

$$ {\varnothing}_{\mathrm{d}}=\frac{\rho_{\mathrm{m}}-{\rho}_{\mathrm{b}}}{\rho_{\mathrm{m}}-{\rho}_{\mathrm{f}}} $$

(16)

$$ {\varnothing}_{\mathrm{s}}=\frac{\Delta t-{\Delta t}_{\mathrm{ma}}}{{\Delta t}_{\mathrm{f}}-{\Delta t}_{\mathrm{ma}}} $$

(17)

$$ {\varnothing}_{\mathrm{e}}=\frac{\varnothing_{\mathrm{d}}+{\varnothing}_{\mathrm{s}}}{2} $$

(18)

where:

V_sh (fraction):

is the volume of shale in formation

GR (API):

is the gamma ray reading of formation

GR_sa (API):

is the gamma ray of clean sands

GR_sh (API):

is the gamma ray of shale

ϕ_d (fraction):

is the density porosity

ϕ_s (fraction):

is the sonic porosity

ϕ_e (fraction):

is the effective porosity

ρ_m (in g/cc):

is the matrix density

ρ_f (in g/cc):

is the fluid density

Δt (μs/ft):

is the interval transit time

Δt_ma (μs/ft):

is the interval transit time of matrix

Δt_f (μs/ft):

is the interval transit time of fluid in the well bore

Water saturation of Archie’s model (Tiab and Donaldson 2004) is shown by Eq. 19–22 in addition to three other shaly sand models, i.e., Simandoux’s model (Crain 2010) in Eq. 23, Waxman–Smits’s model (Darling 2005) in Eq. 24, and Indonesia’s model (Lake 2007) in Eq. 28.

$$ {S}_{\mathrm{w}}^n=\frac{F_{\mathrm{R}}{R}_{\mathrm{w}}}{R_{\mathrm{t}}} $$

(19)

$$ {F}_{\mathrm{R}}=\frac{a}{\varnothing^m} $$

(20)

$$ {S}_{\mathrm{w}}^n=\frac{a{R}_{\mathrm{w}}}{\varnothing^m{R}_{\mathrm{t}}} $$

(21)

$$ {S}_{\mathrm{w}-\mathrm{AR}}=\sqrt[n]{\left(\frac{a{R}_{\mathrm{w}}}{R_{\mathrm{t}}{\upphi}^m}\right)} $$

(22)

$$ {S}_{\mathrm{w}-\mathrm{sim}}=\frac{-\frac{V_{\mathrm{sh}}}{R_{\mathrm{sh}}}+\sqrt{{\left(\frac{V_{\mathrm{sh}}}{R_{\mathrm{sh}}}\right)}^2+\frac{4{\upphi_{\mathrm{e}}}^m}{a{R}_{\mathrm{w}}{R}_{\mathrm{t}}\left(1-{V}_{\mathrm{sh}}\right)}}}{\frac{2{\upphi}_{\mathrm{e}}^m}{a{R}_{\mathrm{w}}\left(1-{V}_{\mathrm{sh}}\right)}} $$

(23)

$$ {S}_{\mathrm{w}-\mathrm{wax}}=\frac{-{R}_{\mathrm{t}}{\upphi_{\mathrm{t}}}^m{\mathrm{BQ}}_{\mathrm{V}}+\sqrt{{\left({R}_{\mathrm{t}}{\upphi_{\mathrm{t}}}^m{\mathrm{BQ}}_{\mathrm{V}}\right)}^2+4\frac{R_{\mathrm{t}}}{R_{\mathrm{w}}}{\upphi_{\mathrm{t}}}^m}}{2\frac{R_{\mathrm{t}}}{R_{\mathrm{w}}}{\upphi_{\mathrm{t}}}^m} $$

(24)

One can estimate the water saturation using the Waxman and Smits model (Eq. 24) with BQ_v calculated by Eq. 25 and C and ϕ_c calculated by Eq. 26 as shown below:

$$ \mathrm{B}{\mathrm{Q}}_{\mathrm{v}}=\frac{\left({\upphi}_{\mathrm{c}}-\upphi \right)}{\left(C\ast \upphi \right)} $$

(25)

$$ {C}_{\mathrm{w}\mathrm{a}}=\frac{\upphi^{-m}}{R_{\mathrm{t}}}=\frac{1}{R_{\mathrm{w}}}+\frac{\upphi_{\mathrm{c}}}{C}\ast \frac{1}{\upphi}-\frac{1}{\upphi_{\mathrm{c}}} $$

(26)

Indonesia’s water saturation model is shown in Eq. 27 and 28.

$$ \frac{1}{R_{\mathrm{t}}}={S_{\mathrm{w}}}^n{\left[{\left(\frac{{V_{\mathrm{sh}}}^{2-{V}_{\mathrm{sh}}}}{R_{\mathrm{sh}}}\right)}^{\frac{1}{2}}+{\left(\frac{{\upphi_{\mathrm{e}}}^m}{R_{\mathrm{w}}}\right)}^{\frac{1}{2}}\right]}^2 $$

(27)

$$ {S}_{\mathrm{w}-\mathrm{indo}}={\left\{{R}_{\mathrm{t}}{\left[{\left(\frac{{V_{\mathrm{sh}}}^{2-{V}_{\mathrm{sh}}}}{R_{\mathrm{sh}}}\right)}^{\frac{1}{2}}+{\left(\frac{{\upphi_{\mathrm{e}}}^m}{R_{\mathrm{w}}}\right)}^{\frac{1}{2}}\right]}^2\right\}}^{\frac{-1}{n}} $$

(28)

where:

F_R (fraction):

is the formation resistivity factor in fraction

R_w:

is the pore water resistivity

R_t:

is the hydrocarbon-bearing formation resistivity

ϕ (fraction):

is the porosity

ϕ_t (fraction):

is the total porosity

ϕ_e (fraction):

is the effective porosity

a:

is a constant for tortuosity

m:

is the cementation factor

n:

is the saturation exponent

S_w (fraction):

is the water saturation

S_w-AR (fraction):

is the water saturation by Archie’s model

S_w-SIM (fraction):

is the water saturation by Simandoux’s model

S_w-WAX (fraction):

is the water saturation by Waxman–Smits’s model

S_w-INDO (fraction):

is the water saturation by Poupon-Leveaux’s (Indonesia) model

B (Ωm⁻¹(cm³/mEq)):

is the equivalent conductance of the clay exchange cations

Q_v (mEq/cm³):

is the cation exchange capacity per unit pore volume

V_sh (fraction):

is the volume of shale in formation

R₀ (Ωm):

is the fully brine-saturated formation resistivity

R_w (Ωm):

is the resistivity of pore brine water

R_sh (Ωm):

is the resistivity of formation at V_sh equal to 1

R_t (Ωm):

is the formation resistivity saturated with water and hydrocarbons