By Tony Moss, NFL Editor -- (Sports Network) - Kyle Orton will begin his tenure as the Chicago Bears' starting quarterback on Thursday night, when Orton and his team welcome the San Francisco 49ers to Soldier Field in the third preseason tune-up for each club.

It was announced on Monday that Orton would be the Bears' opening day starter against the Colts on Sept. 7, and Thursday's contest figures to be a dress rehearsal of sorts for that nationally-televised tilt. Starters are expected to play into the third quarter on Thursday, whereas they will likely play sparingly in the team's final preseason game.

Orton, who is 12-6 in three seasons as a spot-starter, has been good enough thus far in the preseason to wrest control of the top signal-caller job from ex-starter Rex Grossman.

The Purdue product is 12-of-19 for 99 yards without a touchdown or interception in the preseason to date. Grossman, meanwhile, is 13-of-23 for 118 yards with a touchdown and a pick.

Also at issue for the Bears is the running back situation, which was thrown into upheaval when starter Cedric Benson was abruptly released prior to training camp.

Rookie Matt Forte (14 carries, 42 yards in the preseason) and holdovers Garrett Wolfe (12 carries, 69 yards), and Adrian Peterson (6 carries, 39 yards) are all jockeying for position on the depth chart, while former Lion Kevin Jones has yet to appear as he recovers from a knee injury. Jones has a chance to make his Bears debut on Thursday night.

While Chicago's quarterback competition has reached a resolution, the similar situation in San Francisco looks to be concluding also.

Journeyman J.T. O'Sullivan, who has started both preseason games for the 49ers and looks set to open against the Bears as well, appears to have an edge over Alex Smith and Shaun Hill for the job.

The 29-year-old Sullivan, who has never started a regular season NFL game, is 13-of-25 for 225 yards with a touchdown and a pair of interceptions in the preseason to date.

Sullivan is expected to play the first half on Thursday, with Smith relieving him and Hill finishing up.

On the injury front for the 49ers, guard Adam Snyder (ankle), tackle Jonas Jennings (hand), guard David Baas (pectoral), and wideouts Arnaz Battle (hamstring), Bryant Johnson (hamstring), and Ashley Lelie (calf) are among the regulars who won't be available in Chicago.

The Bears lead the all-time preseason series with the 49ers, 3-2, breaking a deadlock in the series with a 31-28 win at Soldier Field prior to last season.