Pre-discovery Data Research Articles

A data-driven approach to estimating post-discovery parameters of unexplored oilfields

Consider a typical situation where an investor is considering acquiring an unexplored oilfield. The oilfield has undergone a preliminary geological and geophysical study in which pre-discovery data such as lithology, depth, depositional system, diagenetic overprint, structural compartmentalization, and trap type are available. In this situation, investors usually estimate production rates using a volumetric approach. A more accurate estimation of production rates can be obtained using analytical methods, which require additional data such as net pay, porosity, oil formation volume factor, permeability, viscosity, and pressure. We call these data post-discovery parameters because they are only available after discovery through exploration drilling. A data-driven approach to estimating post-discovery parameters of an unexplored oilfield is developed based on its pre-discovery data by learning from proven reservoir data. Using the Gaussian mixture model, and a data-driven reservoir typology based on the joint probability distribution of post-discovery parameters is established. We came up with 12 reservoir types. Subsequently, an artificial neural network classification model with the resilient backpropagation algorithm is used to find relationships between pre-discovery data and reservoir types. Based on k-fold cross-validation with k = 10, the accuracy of the classification model is stable with an average of 87.9%. With our approach, an investor considering acquiring an unexplored oilfield can classify the oilfield's reservoir into a particular type and estimate its post-discovery parameters' joint probability distribution. The investor can incorporate this information into a valuation model to calculate the production rates more accurately, estimate the oilfield's value and risk, and make an informed acquisition decision accordingly.

Possible Activity in 468861 (2013 LU28)

Abstract Manx comets are objects on long-period comet orbits that are inactive as they approach perihelion. They are of particular interest because they may help constrain solar system formation models. 2013 LU28 was discovered as an inactive asteroidal object on 2013 June 8 at a heliocentric distance of 21.8 au. Images and photometric data were obtained of 2013 LU28 from multiple telescopes from pre-discovery data in 2010 until the present. Its spectral reflectivity is consistent with typical organic-rich comet surfaces with colors of g ′ − r ′ = 0.97 ± 0.02, r ′ − i ′ = 0.43 ± 0.02, and r ′ − z ′ = 0.65 ± 0.03, corresponding to a spectral reflectivity slope of 30 ± 3%/100 nm. There is no obvious indication of dust coma in deep stacked images. We estimate the nucleus radius to be ∼55.7 ± 0.3 km assuming an albedo of 4%. This is much smaller than the 1σ upper limits on the nucleus size of 79.9 km from the NEOWISE survey assuming the same albedo, since the NEOWISE survey is not very sensitive to objects this small at this distance. The heliocentric light curve suggests possible activity between r ∼ 17 and 13 au where 2013 LU28 is brighter than expected. This is consistent with outgassing from CO or CO2. Using surface brightness profiles, we estimate an upper limit of ∼0.01 kg s−1 for micron-sized dust that can be produced without us detecting it for the inactive portion of the light curve, and upper limits of ∼1 kg s−1 for CO and ∼1.5 kg s−1 for CO2 between 20 and 14.7 au.

The Type II supernova SN 2020jfo in M 61, implications for progenitor system, and explosion dynamics

We present the discovery and extensive follow-up observations of SN 2020jfo, a Type IIP supernova (SN) in the nearby (14.5 Mpc) galaxy M 61. Optical light curves (LCs) and spectra from the Zwicky Transient Facility (ZTF), complemented with data from Swift/UVOT and near-infrared photometry is presented. These were used to model the 350-day duration bolometric light curve, which exhibits a relatively short (∼65 days) plateau. This implies a moderate ejecta mass (∼5 M⊙) at the time of explosion, whereas the deduced amount of ejected radioactive nickel is ∼0.025 M⊙. An extensive series of spectroscopy is presented, including spectropolarimetric observations. The nebular spectra are dominated by Hα, but also reveal emission lines from oxygen and calcium. Comparisons to synthetic nebular spectra indicate an initial progenitor mass of ∼12 M⊙. We also note the presence of stable nickel in the nebular spectrum, and SN 2020jfo joins a small group of SNe that have inferred super-solar Ni/Fe ratios. Several years of prediscovery data were examined, but no signs of precursor activity were found. Pre-explosion Hubble Space Telescope imaging reveals a probable progenitor star, detected only in the reddest band (MF814W ≈ −5.8) and it is fainter than expected for stars in the 10−15 M⊙ range. There is thus some tension between the LC analysis, the nebular spectral modeling, and the pre-explosion imaging. To compare and contrast, we present two additional core-collapse SNe monitored by the ZTF, which also have nebular Hα-dominated spectra. This illustrates how the absence or presence of an interaction with circumstellar material (CSM) affect both the LCs and in particular the nebular spectra. Type II SN 2020amv has a LC powered by CSM interaction, in particular after ∼40 days when the LC is bumpy and slowly evolving. The late-time spectra show strong Hα emission with a structure suggesting emission from a thin, dense shell. The evolution of the complex three-horn line profile is reminiscent of that observed for SN 1998S. Finally, SN 2020jfv has a poorly constrained early-time LC, but it is of interest because of the transition from a hydrogen-poor Type IIb to a Type IIn, where the nebular spectrum after the light-curve rebrightening is dominated by Hα, although with an intermediate line width.

Data analytics in asset valuation under uncertainty: A case study of unexplored oilfields

Valuing unexplored oilfields is challenging since it deals with multiple sources of uncertainty and covers a lengthy period. Previous research suggests that the major source of uncertainty in the valuation of unexplored oilfields is reservoir condition. It is usually represented by a bunch of parameters based on which reserve volume and production rates can be estimated. The values of these parameters are revealed only after exploration well is drilled; we refer them as post-discovery parameters. Prior to exploration drilling, data on some reservoir characteristics, called the pre-discovery parameters, are available. This research aims to develop a model to estimate the probability distribution of post-discovery parameters based on pre-discovery data. The model is data-driven, built using the database of proven reservoirs. We use the Bayesian network to develop the model and apply the k-fold cross validation to test the results. By adding a window parameter to the target variable, the model provides information regarding the trade-off between accuracy and confidence. In general, compared to the initial model that uses a priori clustering based on the reservoir’s lithology and depth, our Bayesian network model produces lower variances.

Characterization of the Nucleus, Morphology, and Activity of Interstellar Comet 2I/Borisov by Optical and Near-infrared GROWTH, Apache Point, IRTF, ZTF, and Keck Observations

Abstract We present visible and near-infrared (NIR) photometric and spectroscopic observations of interstellar object (ISO) 2I/Borisov taken from 2019 September 10 to 2019 December 20 using the GROWTH, the Apache Point Observatory Astrophysical Research Consortium 3.5 m, and the NASA Infrared Telescope Facility 3.0 m combined with pre- and postdiscovery observations of 2I obtained by the Zwicky Transient Facility from 2019 March 17 to 2019 May 5. Comparison with imaging of distant solar system comets shows an object very similar to mildly active solar system comets with an outgassing rate of ∼1027 mol s−1. The photometry, taken in filters spanning the visible and NIR range, shows a gradual brightening trend of ∼0.03 mag day−1 since 2019 September 10 UTC for a reddish object becoming neutral in the NIR. The light curve from recent and prediscovery data reveals a brightness trend suggesting the recent onset of significant H2O sublimation with the comet being active with super volatiles such as CO at heliocentric distances >6 au consistent with its extended morphology. Using the advanced capability to significantly reduce the scattered light from the coma enabled by high-resolution NIR images from Keck adaptive optics taken on 2019 October 4, we estimate a diameter for 2I’s nucleus of ≲1.4 km. We use the size estimates of 1I/’Oumuamua and 2I/Borisov to roughly estimate the slope of the ISO size distribution, resulting in a slope of ∼3.4 ± 1.2, similar to solar system comets and bodies produced from collisional equilibrium.

A search for the origin of the interstellar comet 2I/Borisov

The discovery of the second interstellar object 2I/Borisov on 2019 August 30 raises the question of whether it was ejected recently from a nearby stellar system. Here we compute the asymptotic incoming trajectory of 2I/Borisov, based on both recent and pre-discovery data extending back to December 2018, using a range of force models that account for cometary outgassing. From Gaia DR2 astrometry and radial velocities, we trace back in time the Galactic orbits of 7.4 million stars to look for close encounters with 2I/Borisov. The closest encounter we find took place 910 kyr ago with the M0V star Ross 573, at a separation of 0.068 pc (90% confidence interval of 0.053–0.091 pc) with a relative velocity of 23 km s−1. This encounter is nine times closer than the closest past encounter identified for the first interstellar object 1I/‘Oumuamua. Ejection of 2I/Borisov via a three-body encounter in a binary or planetary system is possible, although such a large ejection velocity is unlikely to be obtained and Ross 573 shows no signs of binarity. We also identify and discuss some other recent close encounters, recognizing that if 2I/Borisov is more than about 10 Myr old, our search would be unlikely to find its parent system.

C/2010 U3 (Boattini): A Bizarre Comet Active at Record Heliocentric Distance

Abstract We present a photometric and dynamical study of comet C/2010 U3 (Boattini), which was seen active in prediscovery data as early as 2005 November at a new inbound record heliocentric distance of r H = 25.8 au. Two outburst events around 2009 and 2017 were observed. The coma and tail of the comet consist of dust grains of ∼10 μm in radius, ejected protractedly at speeds ≲50 m s−1 near the subsolar point, and are subjected to the Lorentz force, solar gravitation, and radiation pressure force all together. The prolonged activity indicates that sublimation of supervolatiles (e.g., CO, CO2) is at play, causing a net mass-loss rate ≳1 kg s−1. To sustain the mass loss, the nucleus radius has to be ≳0.1 km. The color of the cometary dust, similar to other long-period comets, is redder than the solar colors, but we also observed potential color variations when the comet was at 10 < r H < 15 au, concurrent with the onset of crystallization of amorphous water ice, if at all. Using publicly available and our refined astrometric measurements, we estimated the precise trajectory of the comet and propagated it backward to its previous perihelion. We found that the comet visited the planetary region 1.96 ± 0.04 Myr ago, with barycentric perihelion distance q = 8.364 ± 0.004 au. Thus, C/2010 U3 (Boattini) is almost certainly a dynamically old comet from the Oort cloud, and the observed activity cannot be caused by retained heat from the previous apparition.

Seeing Double: ASASSN-18bt Exhibits a Two-component Rise in the Early-time K2 Light Curve

Abstract On 2018 February 4.41, the All-Sky Automated Survey for SuperNovae (ASAS-SN) discovered ASASSN-18bt in the K2 Campaign 16 field. With a redshift of z = 0.01098 and a peak apparent magnitude of B max = 14.31, ASASSN-18bt is the nearest and brightest SNe Ia yet observed by the Kepler spacecraft. Here we present the discovery of ASASSN-18bt, the K2 light curve, and prediscovery data from ASAS-SN and the Asteroid Terrestrial-impact Last Alert System. The K2 early-time light curve has an unprecedented 30-minute cadence and photometric precision for an SN Ia light curve, and it unambiguously shows a ∼4 day nearly linear phase followed by a steeper rise. Thus, ASASSN-18bt joins a growing list of SNe Ia whose early light curves are not well described by a single power law. We show that a double-power-law model fits the data reasonably well, hinting that two physical processes must be responsible for the observed rise. However, we find that current models of the interaction with a nondegenerate companion predict an abrupt rise and cannot adequately explain the initial, slower linear phase. Instead, we find that existing published models with shallow 56Ni are able to span the observed behavior and, with tuning, may be able to reproduce the ASASSN-18bt light curve. Regardless, more theoretical work is needed to satisfactorily model this and other early-time SNe Ia light curves. Finally, we use Swift X-ray nondetections to constrain the presence of circumstellar material (CSM) at much larger distances and lower densities than possible with the optical light curve. For a constant-density CSM, these nondetections constrain ρ < 4.5 × 105 cm−3 at a radius of 4 × 1015 cm from the progenitor star. Assuming a wind-like environment, we place mass loss limits of for v w = 100 km s−1, ruling out some symbiotic progenitor systems. This work highlights the power of well-sampled early-time data and the need for immediate multiband, high-cadence follow-up for progress in understanding SNe Ia.

OUTGASSING BEHAVIOR OF C/2012 S1 (ISON) FROM 2011 SEPTEMBER TO 2013 JUNE

We report photometric observations for comet C/2012 S1 (ISON) obtained during the time period immediately after discovery (r=6.28 AU) until it moved into solar conjunction in mid-2013 June using the UH2.2m, and Gemini North 8-m telescopes on Mauna Kea, the Lowell 1.8m in Flagstaff, the Calar Alto 1.2m telescope in Spain, the VYSOS-5 telescopes on Mauna Loa Hawaii and data from the CARA network. Additional pre-discovery data from the Pan STARRS1 survey extends the light curve back to 2011 September 30 (r=9.4 AU). The images showed a similar tail morphology due to small micron sized particles throughout 2013. Observations at sub-mm wavelengths using the JCMT on 15 nights between 2013 March 9 (r=4.52 AU) and June 16 (r=3.35 AU) were used to search for CO and HCN rotation lines. No gas was detected, with upper limits for CO ranging between (3.5-4.5)E27 molec/s. Combined with published water production rate estimates we have generated ice sublimation models consistent with the photometric light curve. The inbound light curve is likely controlled by sublimation of CO2. At these distances water is not a strong contributor to the outgassing. We also infer that there was a long slow outburst of activity beginning in late 2011 peaking in mid-2013 January (r~5 AU) at which point the activity decreased again through 2013 June. We suggest that this outburst was driven by CO injecting large water ice grains into the coma. Observations as the comet came out of solar conjunction seem to confirm our models.

Discovery and Photometric Observation of the Optical Counterpart of a Possible Galactic Halo X-Ray Transient, XTE J1118+480

Abstract We discovered an optical counterpart of about 13 mag of a soft X-ray transient, XTE J1118+480 on 2000 March 30. We performed astrometry and provided an accurate position of R.A. = 11h18m10s.85, Decl. = +48°02′12″.9. The outbursting object has been identified with a 18.8 mag star in the USNO catalog. Our pre-discovery data shows another outburst during 2000 January, again coinciding with an outburst detected in X-rays. Through CCD time-series photometry, we found the presence of a periodic variation with an amplitude of 0.055 mag and a period of 0.17078 ± 0.00004 d, which we consider to be a promising candidate of the orbital period. Because of the high galactic latitude and faint quiescence magnitude of 18.8, XTE J1118+480 is possibly the first firmly identified black hole candidate X-ray transient in the galactic halo.

Analysis and distribution of hydrocarbon shows in Western thrust belt of the U.S.

Hydrocarbon shows provide a means to evaluate petroleum potential. A trap, reservoir rocks, and hydrocarbons are required for a petroleum prospect. In areas such as the Western thrust belt which have been explored by drilling, hydrocarbon shows can be evaluated from well completion records. The exploration concept would concentrate exploration to define reservoirs and traps in areas that have abundant shows. Abundance of reported shows can be quantified from well records as a show index. A totalshow index is defined as the total number of shows reported on production and formation tests, cores, and driller shows divided by the number of dry holes. A liveshow index is defined as the total number of shows reported on production and formation tests divided by the number of dry holes. In this study, computer analyses of more than 1800 well records were used to compute and map drilling densities and show indexes for four depth ranges and to compare prediscovery data with current results. Comparison of current exploratory drilling densities and show indexes with prediscovery data for the producing fairway in the Wyoming thrust belt indicates petroleum potential for several parts of the Western thrust belt. Show index maps in three depth ranges indicate that shows tend to decrease in abundance and to be concentrated at shallower depths from east to west across the thrust belt. The most favorable exploration trends appear to be along the deep frontal part of the northern Montana thrust belt, along the central Utah hingeline, and in the northern part of the Wyoming thrust belt. Anomalous shallow concentration of driller shows in the Southern thrust belt suggests that a detailed study of resource potential should be conducted before concluding exploration plans in the area. Small show indexes downgrade petroleum potential in the south half of the Montana thrust belt. Widespread geographic and stratigraphic distribution of shows and moderate drilling densities support continued exploration of selected trends with petroleum potential in the Western thrust belt.