Sealing capacity evolution of trap-bounding faults in sand-clay sequences: Insights from present and paleo-oil entrapment in fault-bounded traps in the Qinan area, Bohai Bay Basin, China

Highlights

• We calibrate the relationship between SGR and AFPD based on drilled fault-bounded oil reservoirs.

• The fault sealing capacity is evaluated by the calibrated SGR-AFPD relationship.

• Fault reactivation causes changes in the sealing capacity and hydrocarbon column heights.

• A model to illustrate the evolution of the sealing capacity and hydrocarbon column heights during fault growth.

Abstract

Fault seals have been proven to be a significant risk to exploration success, and methods of evaluating the sealing capacity have been developed in recent decades. However, less attention has been paid to the dynamic change characteristics of the sealing capacity during fault growth. In this paper, the relationship between Shale Gouge Ratio (SGR) and across-fault pressure difference (AFPD) is calibrated to define the seal failure envelope in the Qinan area, thus providing a method for estimating the fault sealing capacity. The sealing capacity of the YEZ fault is evaluated by the SGR-AFPD relationship, which shows that the oil column heights sealable by the YEZ fault (H_seal) are greater than the actual accumulated oil column heights (H_actual). The existence of paleo-oil column is consistent with upfault oil leakage during the reactivation phase, which explains why H_seal > H_actual. As faults undergo multiphase reactivation, the throw accumulates gradually, the minimum SGR of the fault increases and eventually converges to the average V_sh of the entire sequence, which increases the fault sealing capacity during the stable phase and H_seal. In the initial fault formation stage, the minimum SGR and H_seal is small because the reservoir is self-juxtaposed. After fault reactivation, the minimum SGR and H_seal increase compared with that of the initial formation stage because the reservoir slides against more layers of shale. However, if the fault reactivates after hydrocarbon charging, although the minimum SGR and H_seal may increase or remain stable, the H_actual value decreases because of post-charge reactivation-induced leakage. In this case, H_actual may be smaller than H_seal. Thus, although calibrating the SGR-AFPD relationship is effective for predicting the fault sealing capacity, the fault activation phases and hydrocarbon charging history should be clarified first to identify hydrocarbon reservoirs damaged by bounding fault reactivation to avoid overestimating the hydrocarbon column heights.

Introduction

In petroleum exploration, fault seals have been proven to be a significant risk to exploration success (Smith, 1966; Childs et al., 1997; Jones and Hillis, 2003; Manzocchi et al., 2010), and uncertainty in fault sealing capacity often leads to drilling failure or lower hydrocarbon column heights than expected (Bretan et al., 2003; Rudolph and Goulding, 2017). Fault sealing studies have been performed in recent decades, such as on the seal mechanism and evaluation techniques (Watts, 1987; Allan, 1989; Bouvier et al., 1989; Caine et al., 1996; Knipe, 1992; Knott, 1993; Knipe, 1997; Yielding et al., 1997; Bretan et al., 2003; Davies et al., 2003; Childs et al., 2009; Zhang et al., 2010; Meng et al., 2015; Pei et al., 2015; Underschultz and Strand, 2016; Fisher et al., 2017). Faults may seal hydrocarbons in two distinct ways. First, juxtaposition seals, which have been well described in previous studies (Allan et al., 1989; Bouvier et al., 1989; Knipe, 1992; Knipe, 1997), are associated with cases where cross-fault juxtaposition occurs with low-permeability non-reservoir rock. Such a seal generally acts as a very effective barrier to fluid flow. Second, fault gouge often develops along faults, which itself may act as a barrier to fluid flow (Watts, 1987; Caine et al., 1996; Fisher and Knipe, 2001; Fisher and Jolley, 2007; Fisher et al., 2018). But it should note is that the impact of fault gouges on sealing is a contentious issue, some scholars think that this seal mechanism is rare. Due to the small thickness of single-layer mudstones in sand-clay sequences, faults offsetting sand-clay sequences have difficulty forming effective juxtaposition seals, and only those that develop high capillary threshold pressure fault gouges may seal hydrocarbons. The capillary threshold pressure of the fault gouge directly determines the sealing capacity of the fault in sand-clay sequences. The experimental investigations show that the fault gouge capillary threshold pressure increases with increasing clay-gouge volume within a fault zone (Crawford et al., 2002; Cuisiat and Skurtveit, 2010; Giger et al., 2013). Outcrop and core investigations have shown that the fault gouge capillary threshold pressure can be estimated from the clay-gouge volume, maximum burial depths and the depth at time of deformation (Manzocchi et al., 1999; Sperrevik et al., 2002), and confirmed that the capillary threshold pressure of the fault gouge shows increasing trend with increasing clay-gouge volume entrained within the fault gouge under the condition of fixed depth (Gibson, 1998; Sperrevik et al., 2002; Eichhubl et al., 2005).

To evaluate fault sealing capacity, many algorithms that reflect clay-gouge volume within a fault zone have been published in recent years, and they fall into two broad categories (Yielding et al., 1997, 2010): smear factors and gouge ratio. The Clay Smear Potential (CSP) (Bouvier et al., 1989; Fulljames et al., 1997; Lehner and Pilaar, 1997) and Shale Smear Factor (SSF) (Lindsay et al., 1993) consider the smearing of clay or shale beds along a fault surface. The Shale Gouge Ratio (SGR) (Yielding et al., 1997; Freeman et al., 1998; Yielding, 2002) and effective SGR (ESGR) (Freeman et al., 2010) take the average clay content of those beds that have slipped past any point (as determined by the fault throw) and treat it as an estimate of upscaled fault-zone composition. However, all of the above algorithms need to be calibrated in some way because the resultant numbers do not necessarily represent a prediction of the fault sealing capacity (Yielding et al., 1997, 2010; Bretan et al., 2003; Bretan, 2016). Therefore, clay-gouge volume algorithms, such as SGR, must be calibrated against directly measured capillary thresholds of fault gouge or the pressure difference across fault at the same points on the fault (Sperrevik et al., 2002; Yielding et al., 1997; Bretan et al., 2003; Childs et al., 2009). The calibrated relationship is then used predictively on adjacent prospects, and this method is widely applied to evaluate fault sealing capacity and predrill predictions.

However, the result of calibrating sealing capacity only reflects the sealing capacity of faults during the stable phase and less attention is paid to the effect of fault reactivation on the sealing capacity and hydrocarbon accumulation at the field scale. Current faults have grown from fractures or deformation band zones that undergo multiphase reactivation and connection (Aydin and Johnson, 1978; Taylor et al., 2004; Pollard and Fletcher, 2005; Fossen et al., 2007; Schlagenhauf et al., 2008). A fault with a low-permeability fault gouge may seal hydrocarbons during the stable phase, but the porosity and permeability may increase during the active phase, and the fault may act as a conduit for hydrocarbon leakage (Mitchell and Faulkner, 2008; Elkhoury et al., 2011; Indrevær and Stunitz, 2014; Wang et al., 2017). Meanwhile, fault reactivation-induced throw variations also change the clay-gouge volume within the fault zone and the sealing capacity during the stable phase (Reilly et al., 2016). Therefore, the clay-gouge volume, fault sealing capacity and actual hydrocarbon column heights accumulated in fault-bounded traps may present dynamic change characteristics during fault growth, which leads to considerable uncertainty in predicting the hydrocarbon column heights accumulated in fault-bounded traps, thereby significantly increasing the risks of hydrocarbon exploration and the rates of drilling failure.

Therefore, in this paper, we calibrate the relationship between the clay-gouge volume and fault sealing capacity based on current drilled fault-bounded reservoirs from the Qinan area, Bohai Bay Basin, China. The effect of reactivation during fault growth on the sealing capacity and hydrocarbon column heights is analyzed. A model is established to illustrate the evolution of sealing capacity during fault growth and analyze the relationship between the fault sealing capacity and actual hydrocarbon column heights accumulated in fault-bounded traps in sand-clay sequences is analyzed. The aim of this paper is to improve the accuracy of estimating the potential column heights that might be trapped at the fault and the success of exploration.